I've done 360/PS3 -> One/PS4 ports as a contract job before. At a complete guess, there's around six man-months of effort required in porting a large game engine to a new platform, so 12 man-months for two platforms.
Maybe I said it wrong, lets say a big game was made, and was all ready running and for xbox 360 and ps3, how much do you think it would cost to make it play on xbox1 and ps4 ???
A high skilled contractor for that kind of work might ask for $10k per man-month, so $120k total.
Or if you can find someone who can do it in half the time (a 3 month port per platform) and only charge $20/hr, the cost would be about $20k total...
(and that's just the technology -- if your art "isn't HD enough" and has to be updated, you might have to fork out $1M or more...)