A Norris candidate that is a top 5 point producer in the NHL for defensemen that is also above average defensively, is physical and still in his mid 20's with a good chunk of his prime years still ahead of him?
Probably a lot. It depends on the team that is desperate, but there would be a lot. The return would be more than what Jordan Staal brought back. Jordan was never a top 5 point producer, a defensive stalwart sure and a Selke nominee...but Norris nominee carries the weight of "was considered one of the best" if he doesn't win it, if he does, consider his value the type of trade bait that Shero definitely can't bungle up. Not that the Staal trade was messed up or anything. He got back 2 future top 4 defensemen, another that is closer to being NHL ready and a 3rd line guy that is just that, a 3rd line guy as we've all come to terms with.
If there was a guy I would want for Letang, it'd be Bobby Ryan. Maybe. He has 2 more years left and then he's UFA. Not sure about that.
I'm not sure I want to see Letang sign an extension for more than 6m/yr. I'm not a fan of seeing defensemen getting paid upwards of 6+m/yr. Not many are worth that much (Chara, Weber...my list stops there).