Promise is a non-profit organisation. The only goal of requiring relying parties to pay, is to cover the expenses of running the platform.
Relying parties pay for how much they use the platform.
As of september 2020 the exact pricing scheme of Promise is not decided.
The price will be calculated based on each relying party's actual usage of the platform while providing some mimimum threshold so you can get started for free.
The operating budget will be open for everyone to see.