A promise ring shows you are owned... kind of like a collar... only prettier. =O
Now you can say, "hey, woman, I gave you a promise ring -- now your indebted to making me sandwiches whenever I ask!" or something.
Anyways, most promise rings should be around $100 at the most. I don't see what the big deal is when I'm sure most girlfriends have spent more on buying their boyfriends games and different things as a surprise. Then again, if you don't really see the significance of it, I guess there is really no point in buying one for your girlfriend in the first place. *shrug*
EDIT: LOL... WTH is the point of getting a girl a diamond ring from a place like Zales? If you're going to get her a promise ring from a place like that, then just get her something cheap with her birthstone in it -- she and your wallet will equally be happy. There's no reason to spend all of that money on a promise ring; especially from a place that does not produce quality cut diamonds.