OK, so just ran the figures based on Jonathan's design, and by the time you add up all the parts, you'd be looking a finished cost of over £200 per assembly to make them worthwhile making.
That would include the main assembly (Alu shaft, 30t pulley, sealed 7207 bearings), motor pulley, and belt.

However, that's not how I'd personally mount ballnuts , as that's not how they're meant to be mounted.
The precision surface is the external cylinder of the main nut, which should be contained within a hole, but to mount them that way involves scaling things up a bit, however with a bit of a re-design, it could mean they wouldn't need dismantled to swap belts.