So are you then implicitly setting the price yourself because anyone who doesn't charge enough can't get more credits?
Suppose someone develops an app which takes hardly any effort to make -- it's a hundred lines of code -- but it does something common that everybody needs so if available for $0.01 it would have a hundred million users. Which would gross a million dollars and more than pay for the development of the simple app, so the developer is satisfied with that. But to do that you'd have to let them sell a hundred million subscriptions for $0.01 each.
Now let's go toward the other end of the spectrum. Some app which is specialized and requires a million dollars of developer time but only has a market of 10,000 customers. Those customers would pay $100 each for it, if they had to, but not if they can buy into the system somewhere else for $10 (or $0.01) instead.
In general, who is going to buy a fungible subscription for significantly more than it's available somewhere else? How do you handle the fact that the development cost of a thing isn't proportional to the number of people who use it?