One of the best synthetic diamonds out there is Moissanite.
Cool article on Moissanite with pictures and graphics
About the only way you can tell the difference is with a special measuring tool that they show on the third page of the article. It's almost like a diamond in that there are small internal flaws and the color is clear.
You could get a one carat Moissanite that normally would cost 3-5,000$ if it were a diamond for around $500.
There are websites specifically devoted to Moissanite gems and jewlers who specialize in it.
I can get a platinum one carat tiffany copy ring made of Moissanite for around $1200. At Tiffany's it would be around $7000.
Click here to see the copy ring
I never bought a diamond engagement ring for my wife because I don't believe in them. (We have platinum wedding bands from Tiffany) I've thought of buying her a platinum Moissanite Tiffany copy, but I'm not sure how it would go over (no little blue box.)