I spent way too much time thinking about the same thing. It seems to me, if mtaran believes chance of scam is lower than Gwern's, they should both agree that Gwern take the larger payout for smaller chance of being correct about it being a scam.

For what it's worth I do not think that mtaran's original bet is was that great for Gwern to begin with. A $1,000 to $100 bet implies odds of scam is at 9.1%, however Gwern stated his probability is 5%-10% putting the odds at the higher end of Gwern's estimate. If Mtaran is truly confident he needs to offer at least $1,900 to $100 payout for as this will match Gwern's lowest percentage for being a scam (5%).