Given their short career, this year will dictate what their 2nd contracts will be.
If they replicate what they did last season:
Matthews: $8M-$9M
Nylander / Marner: $6M-$7M
If they all better themselves by 10 -20 points :
Matthews (80-90 points): $9.5-$10.5M
Nylander / Marner (70-80)points: $7.5M-$8.5M
Given even a minor improvement on their rookie totals, I can't imagine seeing them sign for less than:
Matthews 10-11
Nylander 7.5-8.5
Marner 6.5-8
And that's not waiting to see what Eichel's new contract is (I'm expecting 9.5+ which will set the floor for Matthews' extension).
Then Pasternak's extension will also color negotiations for Nylander's extension, which I doubt comes in at less than 7x6 and likely is closer to 8x7 given the length of negotiations (reports saying he turned down 6x6). Johansen signed for 8x8 after a 61 pt season at 25. I honestly believe that the Leafs should have signed him already and made an offer for 6x7 before any other notables sign extensions and give his agent grounds to hike his price.
Marner's contract is going to be the cheapest given that he isn't expected to play center, but if he exceeds 70 pts this coming season then he may warrant a match to Nylander's extension.
In short, if the Leafs can sign this trio for under 25 a season, they've done well. I expect to see their cap hit come in closer to 27 and depending on their performance next season, that may climb too.