Affinity is a chargeable product which is calculated based on the account sizes. This is calculated daily and billed monthly. There’s a guide here which may help explain this better than I can.
The Power User Subscription comes with its own features and benefits, but unlike standalone accounts, Affinity connected accounts are not required to have it (but they can still have it optionally). There’s a nice little comparison here which highlights the difference between the subscription and Affinity.
Recently we changed this so that the Affinity account can pay for this if they wish.
All new Affinity accounts come with free starter credit, so I recommend signing up and taking a look around 
I hope that helps, but please let us know if you have any further questions