How many people make ten posts or more per month? Are there eighty? If so, charge $5 per month. When people sign up give them the first month free. They should be able to se the value in thirty days. If nothing else if they return to ask aquestion they will have to pay $5 for that month.
On a personal level I have nothing more to gain. My kids are past playing. But I would pay $5 per month to stay.