I have a different guess. I'm guessing the memory for these are more expensive, and i'm thinking he has to either run a batch of cards with the larger memory-configuration, which means either stopping a production run, and switching reels, or running a extra production run, or, he has to manually remove the memory from finished boards, to replace the memory on them, and then replace them, again, manually. And, then, most likely he has to reprogram em as well. I'm not seeing either as a realistic option, for unknown, and infinitely small production-runs it's fast getting crazy expensive.
Kipper, on the other hand, builds small, dirt simple tech, doesn't pay for machine time to solder much, or most of his stuff, but rather builds to order manually, and call it a personal interest, or a hobby he can make a few buck of if he's lucky. And yeah, i've got nothing but respect for Kipper, and his craft.
There is a huge difference. I'm not sure, but if he sticks to his tweaking, your likely to get a fair bit faster memory from the ACA500, but if you can live with the slower memory, he already said 2Mb chip upgrades most likely will work, so, just get one of those instead?