I paid $1000 for my ring, and it is quite nice, and the lady is more than happy with it. Shop around and look for the ring that is right for you. The whole setting aside this much salary for this amount of time always seemed like a ridiculous scheme concocted and perpetuated by diamond salesmen to me. I know some people that have purchased rings for around $500-$600, and their ladies are very happy with them, too, though I think mine is better.

Basically, if your lady wants to marry you, she'll love whatever ring you give her, and this is how it should be.
Oh, and buy a Canadian diamond! Your girlfriend probably doesn't want to wear a diamond mined at the expense of African slave labourers, and if she does, she probably won't be happy with a ring that only costs $500-$600, anyway.