Neutral fan, Pittsburgh would have to add. It's probably not as much as Boston fans think, but still an add.
Bergeron and Malkin are fairly similar in value. Marchand's value at this point is that of a quality rental player because of needing a new deal after this year. I'd say something like a 1st and either a solid roster player or high end prospect is probably fair.
So the deal would probably have to be something like Malkin, Maatta, 1st for Bergeron, Marchand. IMO that's pretty fair. Malkin is probably the best player in the deal, but his health concerns are real, he makes a ton of money in comparison to Bergeron, Malkin might score more but Bergeron is the more complete player.
So basically Boston gets fairly compensated for a quality UFA winger by getting a 1st and a solid young dman and adds a top flight offensive center, while Pittsburgh adds 2 quality players, one that slots in perfectly behind Crosby as the best "2C" in the game and another that can play on his wing.