In my experience of paying for posting you generally get very poor results unless your clearly stipulate the rules. Try a compatition or something similar instead to encourage members to post
Check the services section, put in a request and you may get offers. The going rate is anywhere from .05 - .20 so its a wide gap depending on the quality of posts you want to receive.
Mostly the rates are 1$ for 20 posts. But if you have specifications for how your posts should be and the quality of posts you want is predefined then you will have to pay more. probably 1$ per 10 posts. Thanks
The common price is something between $0.08 to $0.13 according to your niche. But the price can be changed for advanced writers. Whats the subject of your forum?
alanturco is correct with his advice. You need to monitor what is being posted on your behalf as it has been known for marketers to damage their reputation overnight with some stupid or meaningless posts. Definately use only those with experience which means you'll probably have to pay a higher rate.