From my rudimentary research you can get an optical-quality 1 carat polycrystal disk for $700. Only problem is that it's half a millimeter thick. CVD diamonds

From my rudimentary research you can get an optical-quality 1 carat polycrystal disk for $700. Only problem is that it's half a millimeter thick. CVD diamonds all seem to come in the form of a very thin sheet/disk, so getting something thick enough to use as jewelry may actually approach that price