To enter you need to kill a participant. But the idea of bets is smart.there are ways to go around, eg: you pay up 100 million to enter. Instead of paying it up front you are forced into a bet. If you lose you lose 100 million whereas if you win you can enter.
But a way to make sure the Underworld organisation is in check you must defend your title at the top after every adventure or so. Maybe increase everyone's level up or add some new folks to it.
Daddy-Sama is too lazy to do that. He might forfeit at the final round. 250 is enough T^T