Arbitration hearings tend to use player comparables to try to 'set' a rate for the arbitrator. Naturally, the agent wants the best deal for his client.
Kane was heading to UFA in a couple months. But Kane was 26, while Mantha is currently 25. While he is currently RFA, teams generally pay a premium for those UFA years.
Scoring was totally different on Kane RFA days vs. current days. Comparables are more of guys from same or +1 -1 drafts and same kind of contract status.
Also cap can be anything from the projected one. There's too many variables. Maybe Kane is one, but with very minor weightload.
With 84M cap and without Corona, 7M for Mantha would have been ok deal. As for now, I'm quite sure we don't see anything that kind of figure. Filip Forsberg, Horvat, Hertl and Domi contracts are nearest with age/production.
Quite sure Yzerman can use this situation on his advantage, next season escrow could be high, so he'll offer huge salary on the 1st year or something. Then salary goes down towards the end, but also escrow goes down, so Mantha will get more net money in the end.
Stewie has been a master in these. He will know hoe to please the player for a fair deal, and also get a team-friendly caphit.
In general, think Mantha will work shorter term (4-5 years, still some question marks) and they just should extend Bertuzzi for a maximum 8-year term. Quite sure Tuzzi's game doesn't change anywhere on the long run and would be a steal in the end.