Without actually knowing anything about the subject my guess is that the very cheep synthetic diamonds you heard about were / are intended for tools, (Where all that they care about is the hardness, so they can be small, yellow, and messy)

Also, I do know that you can get synthetic ruby for ~$1/carat on ali-baba. (I actually bought a few)

Maybe that's just the current price point, though I haven't a clue what it would cost to produce one of these things. They're probably trying to capture as much surplus as possible right now.