I mean, "unlocked" iPhones can be worth more money than regular ones, at least in theory. In practice, Apple can probably raise prices by 50% even if they would release the same phone just with an incremented number and people will still buy their stuff.
I’m not sure the DMA works like that. Someone correct me if I’m wrong but as far as I understand the DMA applies to Apple’s operations in the EU, not devices that are sold in the EU. If you buy a “locked” iPhone outside the EU and bring it to the EU and set it up in the EU, I believe that Apple still has to comply with the DMA for that device because all of Apple’s services are still operating in the EU. So Apple wouldn’t be able to charge a premium for “unlocked” devices.