How much would each side have to add

You're trying to swap an unneeded 3rd pair D for a pretty good 2nd pair D.

Needless to say, TOR adds.

Maybe Dermott+Korshkov+3rd for Walker?

no one adds anything. it's even. you could make an argument that Kings add because of age. But aside from age, it's even.

As is, it seems like a pretty even trade to me. Personally, though, I'd rather just keep Dermott because he's more defensive and capable of playing the right.

That will not be the framework because the Kings aren't going to trade a RHD for a LHD. Change Dermott to Kapanen, then you can begin the "who adds" discussion.

In isolation, sound reasonable arguments can be made for nobody, the Kings, or the Maple Leafs. The difference isn't large.

However, in context the Kings aren't doing this because Roy is one of only 3 NHL-qualified RDs they have, and Dermott is one of four or five NHL-ready LDs Toronto has.