how much does it cost to fix a cracked screen on an iphone

It differs on iPhone model because of screen size and type. You should take it to the Apple Store and ask there because I'm not sure if stores differ in price or if the policy has changed since my last visit.

Apple does not nor ever has repaired iOS devices. They exchange damaged items with factory refurbished units (or, very rarely, with new ones if no refurb is available). Since accidental damage is not covered under warranty, you would pay the out of warranty exchange fee to get yours replaced. The out of warranty fees can be found on the web site for your country, under support for the iPhone.