I've been buying shirts at thrift stores for ages but never did the math. I recently cut up a green 'gingham' type checked shirt(third one down). I paid $1.99 for it. It weighed in at (after taking the buttons off) 8 ounces which is about 1.7 yards. I cut it up and weighed the parts...3 ounces went in the trash (I cut off pockets, collars, cuffs, seams and any spots) then the 5 ounces left look usable to me...that is just over a yard since I figure a yard is about 4.5 ounces. Of course every fabric weighs in a little different but 1.99 for a yard of fabric is not bad but I usually do better.
I generally pay .99 for each of my thrift store finds that I'm going to cut up. For example I found this 100% cotton dress with an interesting print (fourth down), paid 99 cents for it and got18 ounces of fabric...13 of it did not go in the trash can. So that works out to 2.8 yards that only cost .35 @ yard. The trick is that if I don't use it I've still thrown that money away.