I don't know the numbers, and I expect none of us does (fair enough, as it's none of our business).
As a general rule, it costs more to get a player in than to keep one. That's life. Everyone says it. Levi will be offered more by clubs who are desperate because players are walking out on them than by North, where he is already and where players are keen to come. It's the same position players at top clubs have been in for a while. Does he want to turn around at the end of his career and be Sam Mitchell or James Kelly, or does he want to be... Actually I can't think of a player at one of the really good teams who has bolted for money before winning a flag first.