How about a two tiered system? Posting is always free as conversation should be free, but you can also pay to pin highlighted content at the top for max 24 hours. 5 or so pinned slots, highest bidder wins. If someone knocks you out of the slots you get paid some back, so you only end up paying for the time you were in one of the slots. Anyone can pay to pin anyone's post too if they think it is noteworthy. Payments are either burned or go towards forum running costs.