Another mechanic you could add is to escalate the cost for the rerolls as the game progresses.
For example, the first reroll of the game is $5 and it goes up by $1 every time anyone pays for one. As you get close to the climax the rerolls will become more important as they become more expensive.
In addition, let anyone (even observers!) buy a reroll (but make sure to let the player who rolls choose whether or not to accept it). Towards the end, everyone can pitch in to pay for the reroll. Force them to move quick and decide, like an auction; then move on.
You may want to do some math based on your expectations of how many you expected people to buy and how much you thought they'd spend. You could keep it expensive if there were other, cheaper, things to spend money on.
Remember, if people are playing for charity, make it as easy as you can for them to give their money away. They want to!