It depends where you go. Mall stores (Kay/Jareds/etc) are all owned by 1 company[0]. Which means you can "shop around" the mall and one may be a few hundred cheaper than the others. But, still all over priced. Stores outside that brand are generally near those stores and also over charge. Both, don't keep many diamonds onsite. They all ask for your budget, and ship in 2-3 stones for you to look at (in your budget). Then, you buy one. Illusion of choice.
If you goto a diamond store (that houses tons onsite) and just go for eye appeal rather than rating. You can get a '15-20k$' mall diamond for a third or less.
For a senior developer in SF area, 3 months salary could get you a loose diamond in the 3-4 carat range off a site like bluenile.com. That's enormous for a ring. In fact, probably so large that disbelief becomes a problem: "That can't be real!"
Wowsers. 1.5 carat solitaire rings look quite huge, 3 carats would be enormous.
I'd be kinda tempted buying one that big for a woman because, you know, if we're going to go for a vulgar display of wealth, then let's make it really f*cking vulgar :)
(I still have better things to do with the money...)
I spent quite a chunk of change on an engagement ring for my fiancée, but it was closer to a week's salary. I think if I'd spent three months salary on it the rock would be so big she wouldn't be able to lift her hand.
I'm making good money, but we're not exactly talking Fortune-500 CEO levels here.