I know three guys that make about the same amount as your boyfriend. They started their own company and so they basically all have the same income. It really depends on the personality. One dude got his wife a 1.25 carat solitaire from Tiffany's, and he put most of his money towards the quality of the diamond. I forget what the clarity and inclusion scale is, but he went for the best. He still only spent $20k-$25k.
Another guy got a 3 stone ring. I think the center stone is about 1 carat and the side stones are about 0.5 carat's and he also spent a lot on the quality and this is probably my favorite of the three rings. I don't know what he spent, but this is a sweet ring. His wife doesn't care about brandname bullsh!t, so he didn't pay the extra ~5k that Tiffany's marks up on their crap.
The last guy to get married spent $25k also, but he bought this thing that just looks retarded. It's 2 carats and 2 carats, in my opinion, just looks stupid. It's too big to be on a woman's finger, unless she has man hands. Which might explain why my mother-in-law's 2-carat ring looks about the right size.
The thing about the first two rings, that I think stands out, is their quality. They always look shiny and catch my eye, more than the 2-carat ring. So make sure you get good quality and you keep it clean. A clean, polished, high-quality diamond looks a million times better than a big diamond.