The current designs are really meant for home fabrication. I'd do things
differently (maybe) if they were destined for a professional fab. But the
fab would be problematic. Most board houses don't do "odd" thicknesses on
small runs (though 0.8mm thick, which is close to the .72 nominal of the
cartridges, is merely "uncommon" rather than "odd.")
For the simplest board (single sided, about 35x35mm, no soldermask, no
silkscreen, no gold) http://www.futurlec.com quotes about $0.90 each plus
$15 setup ($12 additional for silkscreen or soldermask setup. $24 for both.)
They say they do 0.8mm "on request."
That's not bad, I guess. Futurlec has a reputation for being slow and not
very communicative, but otherwise pretty good.
My pcb designs are free for anyone to use, modify, fabricate, sell, whatever.
I don't think "we" have ever nailed down how annoyed matel/etc is likely to
be if someone starts selling (near) clone hardware.