Q: How can I see how much I have to pay?

You can view the account's full purchase history via your computer's iTunes : See your purchase history in the iTunes Store on a Mac or PC - Apple Support

Or you can view the last 90 days purchases via http://reportaproblem.apple.com

Purchases are considered final but you can try the 'report a problem' page to contact iTunes Support and see if they will cancel the purchase  : http://reportaproblem.apple.com

Or you can try contacting iTunes Support via this page : https://support.apple.com/contact or https://www.apple.com/emea/support/itunes/contact.html

If they won't you will have to pay it before you can download anything else.

To try and stop it happening again you can turn off in-app purchases on your phone via Settings > General > Restrictions, and you can also require your password to be entered for all new store purchases via the Restrictions screen