The EA representative told Polygon that the demos should reset to free within the next 24 hours. The representative also noted the error isn't on the PlayStation Network, and we've verified the demos remain free on PS4. When we approached an EA UK representative he wasn't able to offer official comment at the time of publication.
A system error isn't hard to believe considering prior to today the FIFA 14 demo had been free for more than half a year. Also, it's more than a bit odd for it be free on one system and paid on another. In any case, if for some mad reason you're thinking of purchasing one of the demos, wait it out and hopefully we'll get further word from EA soon. Also, if this is an error let's hope EA and Microsoft will be issuing out refunds.
Update: An EA UK representative confirmed to Joystiq the priced demos were "due to a technical error," which has now been corrected. We've confirmed both the UFC demo and FIFA 14 demo have been reverted to free.