Unless Crosby seriously tails off production wise at some point, he is absolutely untradeable.
Since trading for cash or cap space is illegal now, there is simply no trade that could benefit the Pens. From a business perspective they make far too much money from Crosby to ever let him go. From a team success perspective, Crosby is the best player in the world at a more than reasonable salary.
Value wise, I could see two elite rookies still on ELC's plus some additional picks and prospects being fair value, but that still does not address the marketing value Crosby has, and thus, it will never happen.
The problem also becomes one of finances. Sure it's one thing to list 5 current players and ten first round picks, and say maybe that would get Crosby. But what's the value of those players? What's the cost of those players + picks compared to Crosby's salary?
Right now, the Pens are getting a 100+ point scorer, leader and all around dominant player, for $8.7 million. For any deal to make sense, not only do the Pens have to receive equal value (productive players who can score and hustle the way he does), but they also have to combine to make $8.7 million. Otherwise, the Pens are paying more salary for equal or less value.
For example (and just for argument's sake), Edmonton could offer Hemsky, the 1st overall, and Eberle for Crosby. The Pens then have to ask themselves not only whether those three will equal Crosby's production, but how much will those three cost combined when they reach their prime. If Hemsky, Hall and Eberle come close to reaching their ceiling, they're all looking at between $4 to $6 million per year, which suddenly means the Pens are spending $12 to $15 million, just for similar production as they get out of Crosby at $8.7 million.