Be certain that you are an informed consumer when it comes to shopping for a diamond. Several things I recommend is reading up on and understing diamond grading/shape/etc. The two months salary thing is complete bs and certainly not necessary. You may want to find out subtlely how large of a stone your soon to be fiance is comfortable wearing and then go to
http://www.pricescope.com/
and find out how much you should be paying for the stone. I personally would highly recommend buying a smaller quality stone than a larger stone with poor color and fire. I have seen 1/2 carat stones that attract alot more attention than 1 carat stones because they sparkle across the room.
My guess is that you can get a very nice 1/2 carat diamond for between $1200-$1600 depending on the quality. Probably another couple of hundred for the setting and you are set.