I think the demon guy revealed himself because this is a JRPG parody and that's exactly what would happen in a game. Actually this whole thing with the tournament/hero shows up/quest to save the nation felt like it was right off the script for all those JRPGs I played as a kid. And it was awesome.
If each gold coin weighs about as much as a quarter, they'd be about $250 each, which would make 40,000 about $10,000,000. I'm no expert on hiring mercenaries but that doesn't seem ridiculous for hiring what looks like ~60 soldiers and some light vehicles for a day of very heavy combat...