Well…how was he? He made a concept car that cost $82,000. Ummm…he was a millionaire. Even if the prototype cost ten times that much to develop, he would still have had lots of money. Did every invester he had immediatly sell every share of stock from seeing that one car? 
It wasn’t the cost of the car that ruined him, it was its total failure. We’re really not given any details, but maybe he was kicked off of the board of directors of his company, stock became worthless, that kind of thing.
His company was doing poor to begin with, I mean, that’s why he had Homer develop the car in the first place, to put him back on the map.
I assumed that he made more than just the one prototype–didn’t he mention them coming off a production line? That would mean they have a ton of them that they’d never get rid of (plus all the R&D involved).
Earlier in the episode he talked about how his company was being “killed in the marketplace”, but this may have been due to his old-school thinking about producing big peppy cars instead of economic cars named after hungry old Greek broads. The prototype was a desperate attempt to regain market share and it’s failure let the company slide into complete disarray and bankruptcy.
Now let’s not harp on Uncie Herb’s failures without forgetting that the Simpson family was also able to get him back into the black. As far as we know, he’s still making a killing on baby translators. That’s more than can be said for the owner of Mickey Mouse Tanning Salons.
Oddly, Herb managed to avoid the Y-chromosome-carried Simpsons stupidity gene. Maybe the carny part of him conveys immunity.
He did listen to Homer, so he can’t be that smart.
I’ll agree that market and investor confidence are probably responsible for a lot of the slide. Also, it could well be that Herb was borrowing a lot (both in the company’s name and his own,) to stay above water and fund Homer’s “don’t tell me anything about it 'til it’s done” project.
Also, $82,000 was given as the sticker price, which would be (if I understand car lingo correctly,) the starting price for buying one if and when they got mass-produced. (Which indivdual buyers can usually haggle down from.) Putting together a prototype would probably cost a LOT more than that.
Given Herb’s hostility to educated people, I would hazard a guess that he didn’t listen to his accountants and thus managed to piss away his personal fortune along with the company.