You can do this two ways...

1. Fixed price, and try to aim to a value that will cover your targets on a good average. This will drive or divide people to choose between delegations and HBD payment.
2. Floating price, depending on current HIVE price. And assume a standard APR for HIVE power (you should choose what you see best).

There might be other cleaver ways, but this is what I recall. If HBD was possible to delegate, it would be rather simpler right? =)