Large moissanite stones are vastly cheaper than equivalent-quality natural diamonds at trade prices; if you can afford a decent .75ct diamond, you can afford a 15ct moissanite. When cut correctly, moissanite is visibly superior to diamond in fire and brilliance. That's a large part of the reason why small synthetic stones aren't as cheap as you might hope - it takes about the same amount of skilled labour to properly cut a natural diamond, a lab-grown diamond or a moissanite. CZ stones are cheap and look cheap, in large part because they're crudely cut.