I would say 10 for the magnet? Since you usually don't miss more than 10 coins in a run, but might still be useful to get the powerups, even if that's usually what a player goes for anyway.
And regarding the tricks I said it without having ideas, it was more a though. But for example having a coin bonus to encourage the player to keep a certain behavior (example in the example: jumping too often gives you less points since you're repeating the same trick, and you're incentivized to slide on the map instead). But it's a completely new concept, and for the game jam the game works as it is - it's mainly a nitpick on the player having to constantly jump.