You won't find a really good tohru costume for cheaper than 50 bucks online. What i suggest, is trying to make your own.
it could be a lot of fun! and depending on what style you're doing of her, it could be easier than ordering one! look into fabric stores and "how to make" help online. no sense in spending money you dont have to! x_x;; that's just what i would do though. >o< Good luck!