When was the last time a UFA Rental was traded for a teams top prospect? Let alone a 1st on top of that prospect
Basically 100% of the time when you're looking at the rental market for higher end rentals, you're looking at a 1st round pick + a teams ~5th or 6th best prospect.
Now Hall also isn't quite your typical rental either as he is an elite player and not too long ago a Hart trophy winner so he probably gets a tad more than what that typical return is.... But that's also where instead of getting just a couple of assets, it looks like New Jersey is looking to get a basket of solid assets instead, similar to the Avs approach when they dealt Duchene a few years back.
Young Roster player + 1st + B Prospect + B Prospect
That's the type of return it will be. With the young roster player IMO being set up very likely to be Tyson Jost as he really just doesn't have much of a defined role for the Avs anymore.