I think it would depend on a lot of factors. First, what hardware is being used? Does it have a thermostat? What kind? How is the humidity controlled? Can it be used with an auto-turner? How well does it hold temps? Has it hatched any eggs and how well did they hatch?
Then of course it would depend on your market.
I'd be surprised if you could bring more than $25 for a basic styrofoam homemade incubator, and even that might be a bit stiff--I've seen someone trying to sell a Little Giant NIB for $25 without success on craigslist.
But of course it depends on all those factors above. If it's made with a very nice cooler, nice thermostat, and has a proven history, then you might be able to get a bit more. But keep in mind that your market is a very small one, so it might take a while to sell.
I don't think anyone gets rich off making incubators, except maybe GQF & Brinsea. And then only because they sell in enormous quantities.
I don't know if that helps at all, but I thought I'd put in a couple cents anyway.