I'd say covering their losses and maybe foregoing the money Marvel stood to make on the gross, gross of the BO receipts. As a fan, I am concerned about Marvel helping to strengthen the X-Men franchise because I'm not sure Fox, going forward, is going to do a bang up job with their movies. If Fox whiffs on the next couple of movies, X-Men could be in trouble and, if it does get in trouble (think also post Jackman/Singer), it would be better to fork over the $$$ now rather than have to buy back other rights that got traded earlier.
I don't know if any of the above is at all realistic, but please get FF back in the fold.