Booking Pay What You Wish Revenue

We are interested in utilizing the new "pay what you wish" functionality and are trying to figure out the finance-side of things.

If you are doing "pay what you wish", what do you consider the "fair market value" of the ticket?  Do you consider it all earned revenue or is a certain amount contributed revenue?

In our case, we want to set a required base price and then allow for patrons to pay more if they would like; and, ideally we want it to all be earned revenue.

This discussion might be better for the finance forum, however I thought I'd start it here.