OK, there's one person so far who might want one pre-built and tested. I'll need a few more people, I can make a small quantity to start with, but since I'm flat broke I will need to make my money back before too long.

It looks like the cost will be $12 for a hardwired adapter (one system only). Or $13 for the multi-system adapter, with NES, SNES, and Famicom cables being $2 each. That's about what it'll cost me, so that's what I'll charge (+ shipping). I haven't built a Famicom cable yet, but I'm pretty sure it'll work there, too.

It's not as cheap as I'd hoped it would be, but I guess that's not too bad for a small prototype kind of quantity.

So if anyone else would be interested in getting one of these from me sometime in the future, let me know.