I had a great class from a wonderful, wonderful teacher who, in the list of supplies for the class, gave the amounts for a full size quilt and for just a block. Her reason: some people want to learn the technique, others want to make the whole quilt. She also went on to say that if you weren't sure which you wanted to do, to bring scrap fabric for the class. If you really like the block, then you can go purchase your fabric. The worst that happens is you have an orphan block to use for something else. She also gave us "permission" to look at other class projects and if we didn't really like the quilt, to re-purpose the fabric for other quilts without guilt.
Obviously this doesn't work for every quilt class but it sure works for a lot of them. I have followed the lead of others and bought Christmas fabric yardage and use this for my test blocks. One more class and I will have enough for a
Christmas sampler quilt. Another classmate had shades of green, another patriotic fabric.
This idea has been very freeing for me in taking classes.