It may seem silly, but here's kind of what I think about getting engaged to someone. I come from a world where there's no exchanging of property per se, just a more symbolic gesture of giving someone a usually rather expensive ring.

When you get engaged to someone, even if you're not giving them something of physical value, you're offering them yourself, something of great emotional value. Or so I'd hope, if he feels the same way.

But if you're set on something, I think something that's important to the both of you is more effective than something expensive.