At this point, especially with POS only selling around 19,000 copies, it would be hard to guess. Maybe 50,000-100,000 copies if it was just an average game, probably less because of the bad rap from POS? It's really hard to guess, I can't find any sales figures for any Fallout games.
Although with a $10 million dollar price tag just for the license, it would have to sell 200,000 copies at $50 with all the money going to the license buyer just to break even, never mind the development costs or making a profit. If that's seriously the price Herve is asking he's a worse businessman than I thought before, and that's pretty bad.