Offer sheets are weird.
I sort of feel more like GMs have an unspoken code where you just don't offer-sheet another guy's player... unless you want to alienate yourself from the crowd.
And then you have to have a player that is going to also rub people the wrong way by signing with another team.
So... you sort of need two greedy douchebags (I'm being extreme here) to pull that off. (/unhappy, /vengeful, /opportunistic... pick your adjective for reasoning)
Marner doesn't fit the "greedy" bill, so I just don't see it happening.
Last time an offer sheet was signed was ROR in 2013 with CGY - here's article on tiers of compensation.
So... it's been 5+ years since an offer sheets - which sort of suggests something, after there HAD been about 1 per year prior to 2013...
The 2nd lowest offer sheet compensation is $10.1m... so plug that in as your "worst case" - my estimate.
I don't think any team is willing to part with FOUR first rounders... that's just crazy for a winger, crazy.
Personally, I'll ballpark the Leafs as:
Matthews 6x11 = $66m. Same CapHit as Tavares, keeps clubhouse from having the "kid" as the high-money earner - as that can rub people the wrong way over an elite vet like Tavares not being big-dog.
Marner 8x9 = $72m. $9m is more than Draisaitl got ($8.5) and that would be quite a compliment for Marner being a winger - whereas Draisaitl can play two-way center.
My Vegas over/under on Marner would be $9.2m.
Some fun wiki history on Offer Sheets.