I think kps worded his reponse wierdly. The option you have chosen is the one that is going to be in the four-figure range, that is the first option.
No, I mean, getting a one-off set of new spherical keys made would be very expensive. The Round 4 group buy is currently the only realistic way to get new spherical keys for a contemporary PC-10x style keyboard.
As far as old keyboards go, someone has already built a
USB interface for a Commodore 64 keyboard. There may be similar projects for other keyboards. At worst, most mechanical contact keyboards could be rewired similar to
this, although many keyboards older than that directly expose their matrix on the connector. (There were some dirt-cheap ALPS boards in bulk around a while ago, from Heath HTX-10 terminals, and someone within shipping range of you may have one to spare.)