from what I understand about artificial diamonds, the problem is not the cost of manufacture, but the taste of the consumers and resistance from the extant diamond economy, which prevents man-made diamonds from sitting on the fingers of women and bling-addicts.
iirc, there are three factors in this
1. just as there was an initial snobbish dismissal of "cultured" pearls, there is now a snobbish dismissal of artificial diamonds - as if naturally produced rocks are in some way better than man-made ones. This is emotional bias.
2. most synthetic diamonds can be identified as synthetic under alternative light source (I forget the freq.), as they will fluoresce. iirc, some russians are working on a fix for this. last I heard, they had succeeded... and De Beers et alia were NOT pleased.
in other applications - synthetic diamonds of industrial grade are far less expensive to produce than are comparable deposits to mine. however, Boron Nitride in the same adamantine molecular configuration is as cheap and more durable than carbon, so...
addendum:
3. De Beers et alia do what they can to prevent artificial diamonds from gaining a place in the market. And that ain't no small consideration.
I did know that, for cutting and other industrial applications that artificial diamonds were preferred.
As far as jewelry, I don't know that it will ever come to the point that artificial diamonds will take hold. You can purchase a cubic zirconia for a few hundred dollars that resembles a diamond to the naked eye. But young women don't want such a thing for their engagement.