How can I charge people to reply?

How can I charge people to reply..?
I run ClassiPress theme on WP and have recently installed s2Member... I want to leave ad posting free, however I want to charge people to reply to an ad...
Any help would be greatly appreciated.

I run ClassiPress theme on WP and have recently installed s2Member... I want to leave ad posting free, however I want to charge people to reply to an ad...
Any help would be greatly appreciated.
