I'm hoping that you can provide some more information for us so that we can ensure you receive the assistance you need. Please provide the following information:
What Strategic Applications are you using?
What is your exact use case?
We'll try our best to get this answered for you here in this Community discussion, but this does look like more of a Pega Consulting referral. Please let us know if you would like the information on how to engage with them as well.
You should talk to your account team about pricing. I've never heard of us doing a pay per case model. That could be ignorance on my part though. Your sales people would really know the answer to your question.