Buying my girlfriend a promise ring?

I want to buy my girlfriend a promise ring, and this (http://www.lushaejewelry.com/jewelry-jad.asp?p=Item&r=pro-746325) is the one she chose. She chose it solely on look (she didn't see the price). My concern is that the price ($20.00) is very inexpensive for an item of this type, but I have no idea because I've not bought one before. Can someone with more knowledge than me please check this website out and tell me weather or not this is a high quality item, and if not, where I can find one.