It doesn't have anything to do with production costs between versions, it's the new MSRP being $70. Paying $60 for the PS4 version and getting a free upgrade is a loophole they need to close if they want that to become the norm.
Rilcon wrote on Sep 5, 2021, 20:49:
and an opportunity for MS to boast about the way they do things.
I'm sure they're gonna charge $70, too, it's already starting to happen with upcoming third-party titles, even if they're Xbox One compatible.