The way I think of it, if it's over $100, I may as well do it myself. I plan on doing something similar with my Phantom. Also, I believe Unicomp does custom programming.
If you etch the PCB yourself, and reuse the plate, switches, etc, you can get it down pretty low on the DIY keyboards. The only cheaper option is to solder wires from the microcontroller to the switch row/columns individually. I don't know what the key64 cost, but I'm sure it's possible on loss than this (and it doesn't rely on existing hardware) The switches are the most expensive part.
I mean, it's pretty easy to DIY your own convertor box, There's just a microcotroller in there, so they're marking it up a LOT. I looked into this when I first learned Dvorak, and I rejected it as an option.
Edit: Tony: people have bought it. I remember hearing good reviews about it.