Thinking about it some more, if you really want to do this kind of giveaway, then you should structure it as a raffle where everybody gets a chance to win, not just the guy at the top of the leaderboard. Maybe each time somebody finds a ghost, they get a small random chance to win money. That way players don't have to choose between investing ridiculous amounts of time to go for a win and giving up entirely. Each player can increase their chance of a win, no matter how far behind they are.
Just be careful that you don't break any gambling laws.