De Beers specifically ran ads suggesting that you should spend 2 months salary on your ring, and people bought that idea: http://ichef.bbci.co.uk/news/304/media/images/74843000/jpg/_...

When you go to buy a ring they even ask you how much you make some of the time. It's not so much a signaling mechanism, as companies trying to bleed as much as you can possibly afford out of you.