Lots of ideas in the answers here, and maybe one question to answer is: what do my competitors do? Because there are very few questions that are totally new and I'm a believer in not reinventing wheels unnecessarily.
Another principle to apply, in my view, is simplicity. I think many folks are tired of complex pricing models. Bear in mind that transactional pricing can become quite complicated if you have to track those transactions (and potentially police them if your clients are tempted to cheat).
So I'd be tending towards simple pricing - perhaps with a freemium model that sucks your customers in, gets the directory going, builds a client base and track record quickly.

Happy to talk it through if you wish.