I think it depends on the project you're doing. If you're making a showpiece that is going to require a lot of time and effort, and you want it to last for generations, you'd probably want to stay strictly with the highest quality fabric. Most of the quilts I make don't fit into this category, but are made to be used often, and if they eventually wear out, so be it. I try to buy the best when I'm able, but have had to make a few compromises along the way. One thing I do is to look for good quality fabric that's been discontinued and is on sale for $3.00 or $4.00 a yard. This is especially good for backing because of all the fabric it requires. We've all had those sticker shock experiences at one time or another.
Good luck with your class!