Trading any $11M player approaching free agency is difficult. The team dealing him has to view him as an $11M player, and sets the price accordingly. Yet the acquiring club has to view him in terms of his rapidly approaching free agency.
If the market is not a draw (Nashville is sort of in the middle, I think) and the team not already a top contender (same) that is trending upwards (iffy), then there's no reason to expect he'll automatically want to re-up. I'd rate the odds of his signing a long deal in Nashville about one in three. Not great.
If the Leafs deal Marner, the price is going to reflect his production, certainly. Yet it will also reflect the diminishing years under team control. It's going to reflect his intangibles, the percentage of the salary cap he consumes, and perceptions as to his probable interest in staying with his new team.