Couldn't it be possible that NEC/Hudson had a specialized/proprietary solution to manufacture HuCards on the cheap?
Don't know about cheap. But from the way HuCards are constructed, it sems safe to assume that they did have a specialized manufacturing facility to produce them.
How much do we really know about the process they used?
We know they used 'glop-top' (ie, unpackaged circuits) chips. We know they used pressure welding for the chip connections; I believe they used actual gold wire, but am not positive.
This indicates to me that they used industrial robots to mount the chips and connect them to the black carrier board - whatever it may be.
I imagine it's very similar to the way IC's are manufactured today, only instead of being sealed in a plastic case, they were mounted on the board itself, and sealed.
The carrier boards were then glued onto a plastic case, which was (probably) screen printed and packaged.
My gut feeling is that a 'simple' HuCard was not expensive to produce; probably not much more in price than the actual chip would have cost to make, once the set-up costs are paid. However, it would not be cost-effective to do for only a few hundred cards; you would have to manufature 1000's to make it worth-while.
Keep in mind, however, that those are -not- eproms on the card. Those are masked roms, which had to be fabricated just like any other complex circuit. It only makes economical sense if you are producing loads of cards - and, iirc, NEC already had a chip manufaturing plant...
The cost starts to rise when you change the design of the card. Going from 256Mbit (32K) to 4Mbit (512 K) probably wasn't too bad. In fact, I would bet that the larger cards (including some dual-chip cards) were planned for at the start; that's why we have a 20 line address bus. It doesn't take much to have a robot solder 2 chips in, as opposed to only 1.
Things get much more expensive, though, when you start adding other chips in. First, you have to re-design the carrier board - and have them manufactured. It's not like a regular circuit board where you draw in an extra trace, route it, and send it off to be etched. I'm not sure what was involved there, but I am pretty sure it was a big investment to re-design the carrier board.
Then, you have to re-program the robots to handle a third chip (and a second direction of motion). Again, more cost that has to be paid somehow. Not to mention you aren't making cards (or profit) while it is being done
My belief is that NEC saw that making larger (and larger) HuCards was going to require a large investment getting their manufacturing quipment to handle it. At the time, other consoles were being produced that were designed for larger address spaces, and NEC probably couldn't justify the costs in the face of dwindling sales and/or more competition.
[I do think they should have designed the SGX with a two-sided card, though. Put it in a PCE, and it would play; put it in a SGX, and the second board kicks in, giving more space and other options. It seems to me it would have been easier to do.]