It is really the woman's preference and will depend upon the style (heart, princess, etc.); I lucked out that my wife didn't want a large stone, but you can still spend money on quality or the setting. For value purposes, research the net on how to pick a good diamond. If a salesperson has size as their first question, they probably will give you a bad diamond. The priority for value is: Cut, Color, Karat and Clarity. Try a few locations until you find a place you trust and look at a lot of diamonds, they really vary dramatically when you look at them through a loupe. Also, check to see if the store has a Master Gemologist on staff and if they can do appraisals. For this kind of purchase, it may be beneficial to travel to a large city that may have a better selection.

Good Luck.