Precuts are always going to be more expensive per yard than buying your own. The advantage of precuts, as mentioned, is that you get a group of coordinating fabrics from a single line.
If I wanted to buy precuts, I would first find a book I like such as this one for 10" squares:
http://www.amazon.com/Schnibbles-Tim...dp/1564779866/
Find a pattern you want to make, then shop for layer cakes (or whatever other precut you are looking for) to make that quilt. Typically you need more than one jelly roll or layer cake to make a quilt anyway, so then you know beforehand how many precuts to purchase.