I think the tradition is that a man should invest a certain amount of resources. And the point is to not buy/get anything useful. A pile of money for a diamond ring is some resources.
Well, if you are moderately progressive, you still want to be sexists and follow tradition, but you don't wank to sink money in diamonds. I just wanted to say there are alternatives.
But there are alternatives:
This guy made a wirelessly powered led illuminated ring: http://www.kokes.net/projectlonghaul/projectlonghaul.htm
This guy forged a ring out of meteorite metal: http://io9.com/5907849/this-guy-hand-forged-his-own-wedding-...
Those are both, I would say, much larger investments than $2000 to $5000 dollars in money.