How much does it cost to repair iPhone under warranty?

If your phone is under warranty and the damage is not caused by your own personal neglect then it should be covered under the warranty.

You might find this interesting explaining what is and isn't covered:

Our Limited Warranty covers your Apple Display and Apple-branded accessories against manufacturing defects for one year from the date you bought them. The Apple Limited Warranty is in addition to rights provided by consumer law.

Our warranty doesn't cover damage caused by accidents or unauthorised modifications. See the warranty for complete details. You can check your warranty status online and update your proof-of-purchase information if there's an error in our records.

What do you need repairing? If you tell us what you need doing we can assist you further.

Is it a screen replacement? Displays Repair – Official Apple Support

It depends on what you need to be repaired.

Start here:--> iPhone Repair - Official Apple Support

Regards

Giulio