I just cant bring myself to screw people over.
I can certainly see your point, but if someone's offering, and the amount of work you have in the cab would justify 2000$, I can't say you'd be screwing anyone over. When I browse Ebay, I often see the EXACT same item for sale with prices that differ by hundreds of dollars. But I suppose if someone really needed something, didn't have the savvy or patience to search and concluded that a particular asking price was reasonable, then so be it. These are discretionary purchases after all.
On the other hand, if you're, say, a drug maker, you've got a drug that saves peoples lives, and you've already recouped the dev costs on it, and it costs .15c a dose, continuing to sell it for 500$ a dose is flat immoral.
I know, apples and oranges. Point is, people will pay what they feel is reasonable.
As far as selling mame cabs goes, I'd be VERY hesitant to sell cabs loaded with ROMS. Baretta's right, that just puts you out there with a big ol' target on your forehead. esp if you start making decent money at it. Not worth it.
Honestly, I suspect the market for cheaper, low end cabs is already saturated by companies out there, and by guys with just enough know how to do it themselves, so they won't buy a prebuilt.
Maybe there's a market for high end, very avante guard designs that wouldn't look like a huge black refrigerator in your living room. But then again, anyone with enough money to afford something like that (sans roms) would likely already have a game room where a black refrigerator would be perfectly acceptable.
I'd say your best bet is to build what you like, play it for a while, look at what you now don't like about it, take copious notes, then strip it of roms, build another one and ebay the original, lather and repeat.