I agree that there should be very clear, specific rules as to what posts will be paid and which posts will not be paid (I don't see any reason why users can't post in the off topic section for example, as long as they understand they will not be paid for such posts). You should have multiple people (it would probably be too large a job for just one person) check users' post throughout the month and keep track of how many "payable" posts they have made (this would also eliminate the logistical issue of receiving 100's of PM's around the same time each month trying to get everyone paid) every day or every week, allowing for payments to be made automatically.

As for payment structure I might suggest, say a flat rate of .05 for the first 50 posts, then an additional .02375 for at least 75 posts total, then an additional .045 for at least 125 posts total, then an additional .0008 for each post above 125 posts. This would give users a disincentive to spam as they get a less amount on a per post basis after each threshold is met.

In the above example: posts
1 - 50 would be paid an effective rate of .001 per post
51 - 75 would be paid an effective rate of .00095 per post
76 - 125 would be paid an effective rate of .009 per post
126+ would be paid a rate of .008

If you wanted to pay higher levels more then you could add say 20% to hero members, and 25% to legendary/staff members.

I think this scheme (or a similar scheme) would allow you to remain competitive with other advertisers, as the rate for the first 50 posts is on par with the competition, while allowing/encouraging members to post more then only 50 posts, giving you a maximum level of exposure, while also not breaking your advertising budget.