I've previously mentioned elsewhere, that most forums don't pay their moderators as it is considered as a volunteer position. Those forums that do pay their moderators, are on a pay scale, based on how much the owner wants to pay, based on their assumption of how much work is done, and income of the forum
Those that pay without using post to pay as a boost, will generally pay between 15-35 per month. Some forum owners will pay more per month, but it is hard to find those opportunities, as ability does determine the cost. I've even seen some forum owners do a income split with their staff.
Though my personal charge for it if I charge, is atleast $45 to start off, up to $75 per month. This is assuming I am not going to do full time work, but more towards just both moderating and leading.