Pending UFAs are not going to be the centerpieces coming back when sellers trade high-end pending UFAs. Look at first rounders, good-to-great prospects, other futures, and occasionally players signed to term on good contracts (but these are rarer).
The prices are fairly well established as far as the seller-buyer relationships are concerned. Hall is the top free agent available at the deadline (at least it's looking that way, given his apparent reluctance to re-sign with the Devils) and the price will begin with a first rounder, the best one that the Devils can find.
Boston is already behind the eight ball because they are a great team. Their first rounder is going to be late in the round.
The answer is Boston's first rounder plus one of its top three prospects, or something in that ballpark.