http://www.groomgroove.com/the_engagement/two_months_salary_on_diamond_engagement_ring.php
What grooms are buying
At the outset, grooms should know that any indicators of how much an engagement ring should cost refer to the diamond and the setting (ie. the entire engagement ring) and not just the rock itself. That is at least some form of consolation.
Where did the two months of salary idea come from?
The origin of the two months' salary begins with DeBeers, the largest diamond producer and marketer in the world. In 1947, in an effort to increase diamond sales in the United States following a depressed wartime market, DeBeers launched a marketing campaign with a New York advertising firm. A copywriter penned the famous slogan "A diamond is forever". As far as advertising slogans go, it's pretty damn good. A diamond, like your marriage, is supposed to last forever. (ed. - We're thankful wedding preparation doesn't last nearly as long.)
From our research, GroomGroove.com understands that DeBeers wanted to bring diamonds to the masses, rather than have them be for a select few. Accordingly, the price of diamonds was set at approximately two months of salary, which is somewhat tied to inflation.
Diamonds, just like your marriage, are an investment, and have been an investment device for several thousand years. Hard, durable and used to mine for other minerals, diamonds are generally quite rare (although they are not the rarest of gemstones). This, coupled with cartelization by the producers and resellers, keeps diamond prices very high. As a result, it will cost a groom at least $1000 to purchase a diamond of decent cut, clarity, color and carat. Of course, a $1000 diamond engagement ring will likely be lacking in two or three of the four C's. Accordingly, a groom is likely to spend at least $2,500 on his bride's engagement ring.
(Speaking of which - if you want to learn how to make an awesome marriage proposal, we highly recommend our e-book on the subject...)
More for Grooms and the Best Man: Groom and Best Man Wedding Info