I personally wouldn't buy layer cakes, jelly rolls, coordinated precut fabrics, or anything like that at all--these things are much more expensive generally than just buying what you need off the bolt at the shop. If you have a pattern and quilt size generally in mind before you go into the shop, I'll bet if you tell one of the salespeople your budget and what you'd like to make they'd be really helpful in terms of fabric suggestions and how to get the most for your money.
Let us know how your shopping trip comes out!