Bismuth is a tricky metal to polish to the point you can see your face in it. Although it is pretty soft, which aids in the process, it is also quite brittle. Where lapping machines can be dialed in to “set and forget” with bismuth one must go through additional steps of progressively finer grits to avoid the abrasives from simply ripping out material and spoiling the surface being ground. It is largely the extra labor, again, that accounts for the relatively high cost of these cubes.