The price for all iPhones directly from apple is the same.

All sim free devices from apple are factory unlocked and can be activated on any network. You can pop in a ATT or TMO sim and away you go. Vz needs to be activated by either OTA or store.

Carrier specific may, repeat may require you getting prior permission to unlock. So if you buy a ATT device, it may not work with TMO without first getting unlocked by ATT.