If you're just talking numbers, 13M is tops he should get. And he definitely should get less cap % than Rantanen or Drai. Rantanen has sort of similar regular season stats, but blows Marner's numbers out of the water in the ploffs. Plus he has a cup.
It's a little weird to me that Rantanen is just automatically assumed to be the better player. For one, the whole "has a cup" argument is one of the stupider arguments in this day and age given the team aspect of winning a Cup and how difficult it is to win one nowadays. But for now I'll stick to the statistical comparison's between Rantanen and Marner.
Rantanen has a worse statistical profile than Marner in the regular season. Rantanen's career average ppg is 1.09. Marner's is 1.12. Rantanen scores more, for sure (0.46 compared to Marner's 0.33) but he also doesn't play a second of short-handed time, nor is he the two-way player that Marner is. He certainly brings more offense in the playoffs (1.24 in the playoff compared to regular season, which is the sort of jump you want to see).
But like this idea that it's outrageous that Marner would be consider in the same camp cap wise...is weird. It would not be egregious to see them have a similar cap hit.