So let the list of awfulness begin. [This is admittedly skewed to the American experience.]
Kosher meat, which can be even more expensive based on the kind of meat hashgacha you hold by. The price of kosher meat varies greatly depending on where you live and what kind of meat you're looking for. Chicken could be priced almost as well as treif chicken. Lamb, which has lower demand, could be much higher than the secular varieties. If you live outside of New York City or Los Angeles, your grocery store availability is probably not great, and there is probably only one brand, maybe two. You might not even have fresh meat, just frozen. Worse, you may not have access to any meat in your local stores. Either you stop eating meat (and you probably eat a lot less meat regardless) or you locate it somewhere else: road trips with large coolers or order your meat through the mail, probably as a group.
Most people buy glatt kosher, and it seems to be the most widely-available, which can often make it as cheap as kosher meat gets. If you hold by beit Yosef, only a particular rabbi, or only a certain group of rabbis, your purchasing choices are very limited. For example, if you live in Borough Park in Brooklyn, you probably have access to every kind of meat hechsher you could want. However, if you travel to another community for a visit or move there, you likely will not find the unusual hashgachas. Either you bring meat in a cooler, order it through the mail, or stop eating meat. You may have to reassess your priorities and consider whether another hashgacha would be appropriate for your circumstances.
Kosher dairy, especially if you hold by cholov yisrael. Kosher cheese is easily double the price of regular cheese. And you're not going to find the fancy cheeses you loved before you went kosher. There will be a fantastic day when the heavens open and the angels sing because you discovered kosher havarti cheese. Cholov yisrael cheese may not even be available in many smaller communities. Cholov yisrael dairy in general may be nearly twice the price of the stam cholov products, especially milk or coffee creamer.
Kitchen supplies. You need over double (maybe almost triple) the kitchen supplies of "normal" Americans. You probably realize that you will need doubles of almost everything in order to prepare both meat and dairy. Don't forget that many items will also need a pareve option, which will definitely throw you over the mark for "double the supplies" and, depending on your cooking habits and finances, could push you very close to having a kitchen in triplicate. If you can afford it, you will want a dishwasher (or two), two sinks, and two ovens. Kosher kitchen remodels are probably not the conspicuous consumption that they are in most American homes.
The ritual items (Judaica). This is largely an up-front cost, and much of the "nice" stuff might be gifted to you later, most likely for your conversion or wedding. Even if you expect the nice stuff later, you probably still need to buy some cheap stuff to get you by for a while. Some purchases can be postponed. For instance, you don't need a fancy challah/matzah cover; you can use a napkin. If you're fancy, you can buy a $1-2 cloth napkin. Your candlesticks can be cheap ones from Wal-Mart or nice ones from Goodwill. You can use normal cups as kiddush cups and havdalah cups. But this is like eating Ramen in college. It's fine at the time, but eventually, you're going to want to have nice things. (And there is definitely a "keeping up with the Steins" mentality out there.) There is even the mitzvah of hiddur mitzvot to encourage you to "upgrade" your cheap Judaica eventually. But really, you'll just want to look like a normal adult person in your community instead of feeling like a college student. I was always very proud of my first "big girl" Judaica items because either I had saved up for them or received them as a gift, and it was one more marker of being a Jewish adult.
Mezuzahs: This is a big up-front purchase. One case and scroll (kosher handwritten scroll, not a printed one) will cost about $50. The cheapest I've ever gotten out was $40, and the prices can easily go into hundreds of dollars for the very pretty/artistic cases. You'll probably receive many mezuzahs as conversion and wedding presents, but often not enough. And anytime you move into a new home (hello, apartment dwellers who move every year!), you run the risk of needing to buy several more or having half of your mezuzot sitting in a box gathering dust. My inner clutter-hater is very bothered by un-utilized mezuzot.
Books. Most converts are book hoarders. And I might be your leader. Books will easily cost you several thousand dollars just to get a "respectable" home library of basics and some fun stuff. Most of this basic library will be built up during the conversion process, but you'll always be learning, and there will always be more books for your study. The best you can do is watch for sales, buy in person if the middleman mark-up isn't too high, and buy in bulk to get rid of (or minimize) shipping costs. While there are many books you will want to keep a physical copy of, remember that you may want to borrow most books first, either from a friend, library, or shul. For the books you decide have earned a spot in your home, used book websites are everywhere and often have the book you need. If you're patient, you can wait for books to be posted on Half.com or PaperBackSwap.com. I've had surprisingly good luck on PaperBack Swap for Jewish books. (And it's an amazing site generally for all books!)
Clothing. This is primarily an up-front cost. Male or female, you'll probably have clothing adjustments you'll need to make, such as tznius-fying your closet (men too) and buying tzitzit. Clutter-haters may like the spring cleaning aspect of this, but everyone throws out more than they need to. Remember that shells and undershirts can make most shirts tznius. Your booty shorts will unfortunately have to go. But your clubbing tops might not need to. Personally, I also recommend not throwing out your jeans or some other pants until you've gone at least a year without wearing them. It can be comforting to know that they're there, even if you choose not to wear them. You may also want to keep a pair of jeans around for serious cleaning (yay Pesach?), home-improvement projects, or to wear under a skirt while moving, etc. (To be fair, I've moved twice in skirts-with pockets-and it was fine.) Over time, this cost is probably very similar to a "normal" clothing budget, once you've done the initial change-over. However, this cost can go out of control quickly if you don't pay attention to which stores tend to carry tznius women's clothing (men luck out in this department). If you don't, you'll waste gas and time going to multiple stores with zero results, leading to overpriced, frustrated internet shopping binges with high shipping rates. As for tzitzis, they can also be expensive (and itchy), but you can learn to make your own! (Don't try to teach yourself. Just don't.)
Tzedakah: The laws of tzedakah are complex and depend on your individual circumstances. But that is an automatic deduction of your take-home income every year for the rest of your life.
Being shomer Shabbat: Being shomer Shabbat can bring a lot of unexpected (and unpredictable) costs into your life. You will have to live in a certain part of town, and you can't control those property/rental costs. Maybe high housing prices make you decide to move to a different town or state. Little things like nice timers can add up, as well as hot plates (I highly prefer hot plates to blechs for safety reasons) and bathroom tissues. More subtly, Shabbat observance will determine where you are able to work. You may have to take a lower-paying job, and you will miss more work days for chagim. You may not have the freedom to take on a second or third job on the weekends as many Americans do. You may have a harder time finding a job at all, so your periods of unemployment may be longer. You may outnegotiate yourself in job offers because you know you're asking your employer to deal with strange hours and strange days off. If you cause your starting salary to be lower, it's likely that your salary will continue to be lower than your colleagues for years to come. That lower salary could even cause later jobs to offer you lower job offers. (If you think you need to disclose your prior salaries, please read this.) Women are already prone to this self-destructive behavior in the job offer negotiation process, and Shabbat observance may cause you to sell yourself that much shorter.
Wedding. Getting married in the Jewish world is incredibly expensive. Wedding halls, kosher caterers, more fabric in the wedding dress, printing bentchers for the attendees...orthodox weddings can easily cost more than your entire college education. $40,000 is not a shocking number today (even though it is incredibly shocking). $10,000 weddings are considered a bargain. Jewish weddings tend to have many guests, and elopement isn't a popular option. If you're like many adult converts, you will not have family able or willing to pay for your wedding, but you probably also have less guests. However, you may get the "joy" of dealing with 200-300 guests you've never heard of, as your spouse's parents use the wedding to network with every distant relative and business acquaintance. On the other hand, the money the Jewish community is willing to spend on wedding gifts is equally shocking. It's possible that you could actually "make money" on your wedding from the cash and registry gifts. But I don't think you want to rely on that.
Children. Here's the biggie. Day school tuition. Just the thought of it already makes me gag and wheeze. You'll also have to pay for religious celebrations throughout the child's life, such as the bris, bar mitzvah, etc. My observations in college, law school, and my current community make me think that Jewish parents tend to provide financial support to their children longer than the average American parent. (Maybe my data is skewed by being in the South, which has a lot of poverty and values independence? I don't know.) And at the end of the day, you get to pay for a second wedding. And maybe a third, fourth, etc.
I'll go curl up in the fetal position and cry now.