Cash prizes actively put me off because the best thing about a good, well-run game jam is the vibes. People of all experience levels cheering each other on and helping each other out and celebrating the small wins.
When cash enters the chat, you tend to find a lot more kids joining and spamming up the place, and a much less friendly environment overall. Plus, cash jams can grow so quickly that the organisers / moderators are overwhelmed and can’t enforce their own rules.
More participants is not always better.