When fabric was on sale in the past, I bought 4 yds of the ones I liked, because I knew they'd never be available again. Generally speaking, the fabrics on sale were the end of season ones. Sometimes I buy fabric that's NOT on sale that I don't need for a current project, because I like it and I figure it's not going to be around when I do want it for a project. Some designs last, but most of them are only here for a season, so if you love them, you have to buy them when you see them, or they'll be gone forever.
I understand what you're saying about limited budgets, and I AM being careful right now - and have been for the last year or so. I'm mostly sewing from my stash - but I have a lot of left-overs from those 4-yd pieces I've bought over the years at the less-than $9 per yard we're paying now at the LQSs, so my investments in fabric are paying off. I bought them when I COULD afford it!