> First of all, there's no dollar amount tied to how many credits you get for a subscription.

I don't understand what you mean here; their official comms is:

     Customers on existing Plus, Pro and Enterprise/Edu plans should continue to use the legacy rate card. We’ll migrate you to the new rates in the upcoming weeks.

I think it would be more rational to discount a subscription (standard is about 10% in most industries) vs PAYG and agree in principal with your assertion - they haven't specified what the discount is on credits bought in a subscription plan - but there is no indication that they are going to continue allowing thousands of dollars of credits on a $200/m plan.

My guess would be a 10% (or similar) discount if you buy a subscription.