Put it this way: Assume for a moment that Ryan Callahan is a Devil rather than a Ranger. Do you trade Grachev for him straight up? I say you're insane if the answer is yes.
Thats a different question entirely. He's valuable to the organization because he is PART of the organization. He's a young talented player with potentially a long future here.
My ranking of value (outside of obvious franchise players such as Lundqvist) would be:
1) Elite/Superstar, regardless of length of contract (Jagr in the past, Gaborik currently)
2) Talented players with long-term potential (Staal, Dubi and Cally now, in the past you are looking at guys like Graves)
3) Talented short-term vets (Drury, Roszival, etc)
Superstars are valuable because they are in short supply. They are game-changers, and instantly change the makeup of your team.
My group two guys are the core of your team. They may be future stars, or just solid 50 point guys, but these are the guys you retain for ~6-10 seasons if possible who form the heart of your team. Looks at the Devils, and you'd have guys like Madden, Holik, Brylin, etc. Without their core, they don't win their cups.
Third-group guys are important, but ultimately replaceable.
I place Drury somewhere between 2 and 3, he'd my choice for #6.