If we're signing him this off-season, then Chicago, New Jersey and Columbus have set the benchmark. 9M+ is the current rate for your top pairing #1 defenseman, even if he's in the 2nd or 3rd tier in the NHL. Imo, 9M is the tier of Hedman and Makar. 6M+ was the going rate until the extensions of Jones and Werenski and the signing of Hamilton, which has shifted the market massively in their favor.
If I'm Darnell's agent, my job is to maximize his worth and take advantage of opportunities. He had a fantastic season, Norris caliber consistent play that has taken a step in almost every facet of his game. But we've seen it happen countless times, what if next season isn't so pretty? What if regresses a bit in some areas, struggles at times or has injury trouble? Now his worth has declined. What if the market corrects itself and the going rate goes back a mil or two? All scenarios that are realistic. So if I'm his agent, I'm telling him to extend now. If they're willing to pay 9M+ on an 8 year term, this is an opportunity you cannot gamble with. In 5 years (1 more UFA year and the rumored 4 year contract), he'll be 31. We don't know where his game will be at by then, and I doubt teams will want to commit 9M+ long term to a 31 year old defenseman, that's probably not in that 1st tier (yet).
Imo, 9M+ AAV would be an overpayment, but not by much. And we know the supposed added premium needed to play in Edmonton. If the market remains the same, there will be handfuls of teams that would be willing to pay Nurse that 9M+/8yr deal. He's an asset we cannot lose, and I hope we hear about an extension before the seasons starts, so there isn't any drama or speculation to distract him, or us. Prediction, 60M/7yr extension (8.571M AAV)