I don't "stock up" or stash fabrics really so I buy for projects and look for what I need for them. I have no qualms about paying full price for something if it is really worth it to me. I also won't buy cheap fabrics just because the prices are cheap. To me that is false economy. However, when I find really quality fabric on sale, that's a huge bonus!
I'd say my average cost of fabric has been around $15 a yard, but bear in mind I make a lot of clothes too- baby things, dresses, swimsuits for my girls, etc. and those fabrics tend to be pricier than quilting cottons. Not always, but usually.