This summer I bought a dark green duvet cover at a $5 bag rummage sale. I rolled the cover very tightly, and still had room to add a couple more sheets and some curtains. I used my dearly beloved Fons & Porter seam ripper and now have quite a bit of usable fabric for just $5. The two greens (duvet cover) look brand new. One set of curtains ended up as the lining and pockets of the mat & ruler carrier that I made.
Last year, I bought a slightly used Dan River king sized flat sheet for $3. (Goodwill) Made myself a very cute summer dress, using the top ruffle of the sheet as the bottom ruffle of my dress.
Another collection I've got is from bed ruffles that I usually pay about $2 for (thrift stores). I buy the eyelet and lacy ones------------rip them apart and have plenty of 12-16" wide eyelets.
OHHHHHHHHHH and the $2 white eyelet shower curtain that I made into a light weight jacket. (church thrift shop)
Also, our Goodwill store sells fabric for 50 cents a yard, if they have it. Got to watch, since some of the clerks don't have any idea what a yard is. Just got a kid's vest panel for 50 cents last week.