I think you just nailed it, not the Sandin part, but in general the problem in making this deal is that while Petry is attractive to the Leafs, they don't really have pieces that work for this type of deal. They have their start type core, which obviously aren't moving, and they have depth pieces, which Montreal isn't interested in.
Toronto's best prospects that fit the right value for Petry would be their young D prospects, but Toronto is unlikely willing to part with them simply because it's an organizational wide weakness so it's really hard for them to trade from there.
If Montreal gets to a point where they are in it mostly for picks, then maybe Toronto becomes a reasonable partner, but given that they are divisional rivals, and chances are teams outside the division actually have better pieces to create a trade with, it's probably unlikely to make a reasonable trade.
Edit: Meant to comment what I meant by "not the Sandin part", because I meant I don't think the Leafs will trade from their D prospects, but not because Petry isn't worth that kind of return, but simply because when a team has an organizational weakness, it's hard to trade your prospects in that area for a guy over 30. I didn't mean it sound like a sleight against the proposal that Sandin is the type of value the Leafs would have to offer up, I agree with that, just that I think because the Leafs are unlikely to offer something like that is the reason a Petry trade likely happens elsewhere.
