Yep, all good stuff said here....
Definitely
The folks that are serious, seem to have a good amount of Disposable income. Build one [and] try to get it somewhere that other folks (rich folks) will see it, and say "I want one of those".
Yep, this is best way to make any cash out of it, as far as I can tell.
Kinda do an estimate for someone to gauge their seriousness, then work on a pay as you go basis, having them front the $$ for the supplies while you do the sourcing, figuring, and building.
This is how I'm doing it with some friends. Our secretary at work really got excited when I started showing people my first cabinet a few weeks ago, she's convinced her husband that they need me to make one for their kids, etc. So, I made her take her fam to a nicklecade to help her and her hubby decide on just what type of games they want to be able to play, and then I gave them a reality check on some of it, etc. Then I created a price-list for them of what it's going to cost (w/ a line item for my cost of labor, etc), it's a bit more than what they we're hoping it'd be, so now we're just trying to massage their list of "wants" more into what they "need" and hopefully I'll be off and running. But, no, the profit won't be tremendous, just enough to make it worth my time, and maybe help recoup a bit of the cost of my own (ok, no, I've got way too much into my own for that to happen

).
The other extreme is another friend of my wife's who came over to our house and saw my machine. Now her and her hubby really want one, but they want the absolute top-o-the-line machine. She basically wants a duplicate of mine, with all the animated LED's, etc. I told my wife to let her friend know that a duplicate of mine would cost an insane amount of money, because I'm not doing all that soldering and wiring again without getting some serious cash out of it!

So, I'm still not sure where they'll end up, but they're a very affluent family (million dollar house, and all that (even here in Utah!)), so they might still want to convince me to do it. Now, that one will be the one I'd make some cash on, if it happens.
Anyways, so yeah, in a nut-shell, everyone that seriously wants one either wants to build it themselves, or else they usually don't want to spend the money. So, if you want to make any money at all, you're going to need to aim for the high-end market where people don't blink at the idea of spending upwards of $2-3k for their own arcade machine, but the last thing they want is to build it themselves.