I rarely buy anything that's not on sale, unless it's at a Mennonite shop where prices are still about $10/yard.
However, if it's the perfect fabric for a project, I might splurge. Last year, I found the perfect background for my Sunnyside Up class. At $16/yard, it was the most I ever paid for fabric. And I needed 6 yards, or so the pattern said. I didn't use it all, but I do love it, and have been using it in other projects.