I dropped 3-4 sizes (where going from a 10 to an 8 is one size down) over the course of 7 or 8 months. I found that I was able to wear some of my old clothes, which granted were a little big. Tops are easier than bottoms.
I shop at Tablots pretty exclusively so buying a wardrobe is pricy. So what I would do is go in and buy a few pants on sale (try to couple a say with a coupon day and you can get $100 pants for $25) and get them a little small. Usually within 3 weeks they'd fit well. And I'd be able to wear them for a while before they got too big. Then I'd repeat the process. Right now I am wearing some pants that are a little big but I have pants that fit properly. Really I'm spanning 2 sizes right now although the smaller size is my proper size.
The other option is to have some of your clothes taken in. You can usually successfully have clothing tailored to one size smaller and then wear the taken in stuff until it is more than a size too big on you.
You'll find it's not as expensive as you think that it is. Last year I spent just over $1000 on clothing but the what I bought at the end of the year will likely be able to stay in my closet "forever." In other words, I only lost $300 or so in clothes that I can't reasonably wear now. I also know that I will gradually replace the stuff that is a little big on me over the next year or two since that's how I wear out clothes. I know that it is unlikely that I will get three years out of a shirt/pants before it shows it's age or gets stained.