It seems that sooner or later, you are going to have to face this question if you are undertaking a major weight loss project--whether by surgery or not. After all, you have to stay covered. First off there is a practical matter--jeans simply won't stay up if they are 3 sizes too big, and stuff starts to look ridiculous. Then you have to decide how important your clothes are. This is a big question for anyone who has been morbidly obese; after all we don't have a lot of selection or choice when we outgrow the "normal" sizes in the stores. If you have a job where you have to keep up a certain dress code or appearance, you are going to have to budget in some new clothes in sizes that you might not live in very long. Consider this when you decide if you can afford WLS; we don't get to jump from size 20 to size 6 overnight, no matter what your friends might think. Still, there are ways to make this a little less painful and expensive.
I think it helps to decide what is the most important thing in your wardrobe. For me, while I was in active weight-loss phase, it was very important to me to have jeans that fit. They didn't have to be expensive or any designer brand, but they had to fit, so I committed to buying 1-2 pair of Old Navy jeans in a cut that flattered me in each size. That was 5 different sizes once I started losing weight, at $30 per pair (unless I found them on sale). Not an insignificant expense, but one that I decided was worth making. I spent anywhere from 3-6 months in each size; for people losing more rapidly, this might not be practical.
Beyond this, I stuck to a few basic rules:
1. Skirts are forgiving, and often can be worn simply lower on the hips when they get loose, or can be altered.
2. Dresses in knit fabrics work for a lot of sizes.
3. Knit fabrics in general allow for a lot of use while losing weight.
4. A good belt goes a long way.
One thing that I don't think some people plan for is the fact that buying clothes becomes dangerously fun once you really lose some weight. Even when you aren't near goal, just looking and feeling so much better, and being able to fit into smaller sizes and new brands, can make shopping addictive. I think it helps to set your ground rules and your budget for transition size clothes. I accepted the fact that I would buy some clothes that I wouldn't wear very long, and tried my best to hunt for bargains and minimize wasting money.
Some things that people losing weight have done to keep things cheaper include:
-shopping cheap stores and sales (duh, I know)
-exchange clothes with other people losing weight--message boards like LBT or OH are helpful, and others I'm sure, as well as support groups
-thrift shops and consignment shops
-having a good tailor alter clothes that are too nice to just give up. Some items are too complex for this, but having a waist taken in on a skirt or pair of pants is pretty simple. This isn't necessarily cheap, though, so I save this for really nice things I can't part with. If you can do it yourself, great--but even though I'm a pretty good seamstress, there are many things I'd just rather pay a pro to alter.
-Ebay can be a big help, or not. I have found that I can do okay when I am looking for very specific things, but if you frequent Ebay, you know not everything there is a good deal. If you like to sell there, you can unload your stuff that no longer fits. I don't have time for Ebay selling these days, but I used to do it, and it's okay--it never made me so much money to make it really worth putting the effort into.
A lot of this is kind of obvious, I guess, but I think just thinking about the issue of clothes is important when preparing to lose significant weight (or once you've started and realize the problem). Shopping in your own closet is fun, digging out old things that ceased to fit when you gained weight--but for most of us this only takes us so far, until we either surpass our previous lowest weight and run out of old things, or get to the stuff that is just too old and out of date to wear. I eventually got to the old shirts that I bought several years ago that did fit again, but were purchased in that (painfully long) era when all shirts were shorter, even if they weren't midriff-baring exactly (and how happy I was when they finally started making shirts a little longer again, more suitable for us larger-breasted and larger-bellied ladies!). I was so happy to be able to buy shirts that were a longer length and toss those shorter ones to Goodwill.
I discovered some interesting things when I went through this process, too. For example, I found a lot of skirts that I liked but were just a little shorter than I was comfortable wearing nowadays--but if I bought a size or two larger, it would just sit lower on my hips and appear longer, and I could just wear a top over the lower waist. My waist has always been significantly smaller than my hips, so this works great for me. I also discovered which styles of inexpensive knit dresses were easily taken in at the sideseams to make them fit when I got smaller. Overall, my wardrobe became a lot more versatile as I had to change it around to accomodate my changing size.
Shoes and purses and accessories are the main shopping obsessions of obese ladies who like to shop but don't like to suffer the indignity of shopping for clothes. I found as I bought more clothes in transition sizes, I had to stop buying shoes and purses. I just couldn't afford it. Luckily, I had a lot of years to collect nice ones, so it wasn't that hard to do. I'm a lot more intentional now about buying things in general--I shop for a specific item that I need (especially now that I don't have my own income). I'm also lucky that my shoe size didn't change when I lost weight; this does happen with some folks, especially if losing more than 100 lbs total.
Underwear becomes an issue, too. I'm addicted to Costco's Itsee Bitsees panties, and bought them in mediums, then smalls. So I have about a million pairs of them in 3 sizes total; oh well. I have one style of Victoria's Secret bra that I love, and have had pretty good success getting them cheaper on Ebay. Don't forget to be refitted. The sisters shrink for most of us when we lose weight; we need to give them as much help as possible by making sure we wear bras that fit. Especially if your shirts aren't perfectly sized, it really helps to wear bras that fit underneath. I know this is a challenge when you are starting with very large bra sizes. Years before my WLS, I had breast reduction surgery, and I remember how expensive those H cup bras are. (Once I reached "H", I only bought one bra, and immediately pursued breast reduction surgery. The bra was $60 back in 2001.) So this might not be practical to keep up with perfectly if you start out a 44J or something. But again, Ebay can be your friend here, especially if you have a specific brand that consistently fits well for you. I've heard some people swear by some of the bras at Walmart; I don't shop at Walmart myself, so I can't help you there, but if you do, I'm sure you can save a ton of money doing that. Just be sure you know how to fit it properly.
What do you do with your bigger clothes? Well, for most things, I actually have kept them. The reason is that I hope to be pregnant in the next year or two, and I think I'll be able to wear a lot of it as maternity clothes. After I am done with childbearing, I will get rid of the big stuff. Things that I am pretty confident I won't be using again, I donate to Goodwill. If you are a part of a clothing exchange group, you're all set. And if you have really nice plus-size clothes, you can make pretty good money on Ebay, as that stuff is hard to find and a lot of us hated shopping in stores when we were morbidly obese. Consignment would work well for the good stuff, too. The undies are a loss, but that's why I don't spend a lot of money on them. The bras should be donatable, or possibly resale-able if in very good condition. Unless you think you have a good reason to need bigger clothes, like an upcoming pregnancy, it's usually considered best to let go of them when you reach goal and are maintaining it for some time. I had a few times before my surgery where I lost weight and thought I'd never regain, got rid of things, then had to buy them again, in shame, when I regained. So I would not be the one to tell you to chuck it all as soon as you shrink out of it. I'd make sure that you are really maintaining, at least within a few sizes of wherever you end up. But some people would say that is just allowing yourself to regain. Choose for yourself.
I hope this helps pre-ops and non-surgical folks (or recent post ops, or anyone really) at least in starting to think about how to tackle the problem. Nothing about major weight loss is cheap, although I'm sure there are ultra-frugal ways of doing it like there seems to be for everything else, if you want to put in the effort. Having a game plan and sticking to it is what worked best for me.