Castro is a very good and very young SS. He has his limitations; namely he doesn't walk and his glove at SS has been pegged as below average by UZR so far in nearly two full seasons. Still, I believe he's athletic enough to stay at SS for several more years and has a solid bat. Plus, he has tons of upside.
Castro produced 2.2 WAR in 2010 over the course of 125 games. He improved last year, posting a WAR of 3.4 in 158 games. That's durability and a talented player. What's even more impressive is he's 21 until next March, meaning he put up that 3.4 WAR as a 21 year-old at a premium defensive position. Players like don't grow on trees.
He's super 2 eligible (which I'm not sure how that will apply), and will be arbitration eligible next year. That means he's under control through 2016.
What it'll take
As for what it would take. I'm not sure if the Cubs would want a SS back, but it wouldn't hurt to dangle Mercer or D'Arnaud as a start, mainly because they would presumably lose value in our organization as they would be blocked. Considering the Cubs no longer have Lee (traded to Tampa), their SS is in question, so one of the two SS's could be the first piece.
However, the one chosen wouldn't even be the main piece. The Cubs are interested in pitching depth. Cole and Taillon aren't going anywhere. Heredia is interesting. Obviously the first thought is it would be insane to move someone with such potential, but he just finished pitching in the GCL. He's 17 and has a high bust rate, so getting a SS for the next 5 years would be very valuable, even at Heredia's expense. From that same perspective, Epstein would probably balk at just Heredia and Mercer/D'Arnaund. We'd probably have to sweeten the pot. Maybe throw in one of Dodson/Cain/ZVR and one of Locke/Owens/Lincoln/McPherson.
That seems like a steep price, but we are getting a 4+ WAR SS for the next 5 years. Castro isn't as good as McCutchen, but his value really isn't that far off, and the Pirates would demand two top 50 prospects back.