How to set pricing

Greetings everyone,

I am a software developer with an entrepreneurial bent and I have been wanting to develop software on my own for resale purposes for some time now. I've come up with an idea of a web based service that helps real estate agents get feedback after their listings are shown.

My problem is this: I have no idea how to price the service. I could charge per realtor, per month, per listing, per year, and on and on...

Does anyone have any suggestions on how to proceed? What has worked for you guys in the past?

Many Thanks,

Jack Lunn