The spammers wouldn't pay it once though - the idea is that it's a good way to scale moderation. Each time an admin needs to ban a user there is a 10$ subsidy supporting that action - and if the bots come back then they get to pay 10$ to be banned again.
Assuming the money isn't wasted and is actually used to fund moderation 10$ is probably comfortably above the cost to detect and ban most malicious users.
