It's been a while since I checked this thread and haven't read through the whole thing, so forgive me if this has been said before.
Check craigslist and eBay. I just bought a ~.50ct diamond ring (I just wanted the diamond, gonna sell the ring and set the rock in something nicer) for my girlfriend earlier today on CL. Only cost $500. Should be able to bring that down to ~$400 after I sell the ring. FWIW, it's around G SI2. Should have cost twice as much.