I don't really collect fabric in the hopes that I'll figure out what to do with it later. I usually only buy what I need for whatever project I'm working on. Occassionally I will buy something outside the project realm if I'm at a fabric store out of town and fall in love with a fabric I HAVE to have and I don't know I'll ever see again (like the denim with the embossed velvet filgree design).
So if I had $30 I'd buy what I need. If there was any left over I'd see if I could buy something that I liked.