I don't think Leafs can afford to pay Marner 12 million which would likely mean a 7 year deal. I would assume they will end up signing Marner to a shorter deal similar to what they just signed Matthews for. I think over the last 2 years his play and point totals shows he is at least at a similar level to Matthews and Taveras (some could even argue this year he has been the Leafs best player).
Leafs I believe are in trouble in that they are so popular their players tend to get over rated compared to similar level players on other teams. Clearly Tavares took no home town discount, neither did Matthews or Nylander. Leaf fans can hope Marner will but I think his agent is doing a poor job if he signs a contract below Matthews or Taveras level. Marner likely sells more Jerseys than someone like Kucherov who scores more points and that value and extra revenue he brings in will likely bring him extra salary. It probably contributed to Taveras, Matthews and Nylanders high contracts.
I think the Leafs will have a top 4 that will be competitive with any top 4 in the league but I also think they are paying more for this top 4 than other organizations and could hurt future cup runs.
I see Toronto in hockey similar to the New York Yankees who tend to pay more to get players because of the extra publicity associated with playing in New York. The major difference being baseball doesn't have a salary cap but the nhl does.