:edit: I wanted to add that I believe I want a dynamic pricing scheme like Heroku's where they have a slider for how many dynos you want. The problem I run into is that I have a couple metrics that I can charge for, I just don't know how to meld them together.
Intercom.io is a brilliant example of this. They could have priced a million different ways, but they chose to charge by active users on their customer's apps. Now I don't even care about the price, I'll be happy to be doing so well each time I go up one of their price bands!
"We think getting your whole team on Intercom is a good thing. And we think talking to your customers is a good thing too. So we won’t tax you for either. We charge you more as your user base grows. Our interests are aligned."