if you are too picky about who gets to participate then you will not be able to fill your budget which means your site will not get the exposure it is trying to pay for. You should offer pay per post and have strict criteria as to what is counted as a constructive post and make this criteria public. Your criteria should be such that anyone can look at any members posts for the month and realistically be able to say how many posts they should get paid for.

If you pick users to participate in any way other then first come first serve then people might think you are using your shills/Alts to participate plus it would not be transparent.

I would make it competitive with prime dice's old rates with potentially a lower rate after someone has made a lot of posts (so people will not try to spam as much).

I would say .0014,.0012,.001,.0007,.0005 per post for legendary, hero, senior, full and members (when they apply) for the first 250 posts then lower each rate by .0003 for posts after that up to 500. You could potentially make the limits lower for lower ranking users as they tend to spam more.

If your campaign will last more then a month then you should kick out users who make a lot of low quality posts for at least a month.