You can see how much you'll pay for your next Series credit in your Series settings.
Open the Chatbooks app
Tap into your Series subscription
Tap the Settings (gear icon) in the upper right corner
At the top of the screen, you'll see the cost of your next credit
This is what you'll be charged for your next book credit. Each credit prints one book.
If you're in a shared Series, this is the price for your own subscription.