I'm too lazy to link but apparently Jackman said, while publicising Prisoners, something along the lines that it would take a really special script to get him to reprise the role again. Personally, I'm taking that with a pinch of salt.
Sounds like possible leveraging. Could be he's angling for a stronger position at the bargaining table. Maybe any contract he had is over with DOFP, so a third Wolverine and/or other appearances will mean FOX has to pony up.