I went through this question last year myself when I got engaged. My husband (then fiance) and I agreed that the 2-3 months' salary the diamond industry suggests one spend is insane. He actually would've spent 2 months salary if that was the type of ring I really wanted, but I would feel uncomfortable walking around with that much cash on my finger (not to mention we were soon to move to the UK, so that debt would be hanging over *both* of us!).
We decided that $3,500-$4,000 (U.S) was an acceptable price, and I think we spent $3,600 or so on my engagement ring and $200 on the wedding band (I would've been for spending half that much if I were looking for white gold, but I had my heart set on platinum). I spent $500 on his platinum wedding band. My ring has a .60 carat main diamond and two smaller diamonds on either side, and then three more tiny diamonds on each side. It's gorgeous and sparkles like mad. I originally wanted a main diamond just under 1 carat in size, but that really pumped the price up, and I wanted a pretty, clear diamond with no visible flaws. The one I got is just the right size for me, and I do have my carat in a way, as the ring is just over one carat when you total all the diamonds in it.
Basically the cost should be what you can afford, and what you can live with. Get something you'll love and don't pinch pennies too much, as it's an important purchase, but also be wary of thinking that "I'll be wearing this forever so it's worth the price even if it means we'll be broke for years until it's paid off." I personally would feel funny if we had gotten a $10,000 ring -- think of all the things we can do with that $6,000 we saved!