If it is to beef up a stash, look for basic colors in small scale prints and solids so it will be most flexible in future coordination. That is my rule of thumb.
Buy what you can COMFORTABLLY afford, but by only what you like the best and think you will honestly use in a quilt sometime. I try to get at least 1 yard and try to shoot for 3 or the end of bolt if it fits the budget.
And buy the special ones that you just can't take your eyes off of :-)