You can check how much you're paying for a subscription credit anytime in your subscription settings.
How do I check my subscription price in the app?
Open the Chatbooks app and tap into your Monthbook or Monthly Mini subscription
Tap the Settings (gear icon) in the upper right corner
Tap Manage Subscription
At the bottom of the screen, under Billing cadence, you'll see your current subscription price
This is what you'll be charged for your upcoming book credit, which you can use to send a book to print.