I believe you should invest in good quality fabric for your stash.
Poor quality fabric produces poor quality quilts that will not last long. As well as can be discouraging to work with because of the poor quality of the finished quilt. Poor quality fabrics stretch and ravel, making it harder to produce quality quilts.
Check to see when your local quilt shops have sales.
Purchase 1/2 yard pieces. Concentrating on the kind of quilts you intend to make. (crib, children, lap, etc.)
I would rather have fewer yards of good quality fabric than more yards of poor quality fabric in my stash.