Most recent comp is the trade of Michael Bourn last year. The Braves did not have to trade any of their pitching prospects to get him. They sent Jordan Shafer, Brett Oberholtzer, Paul Clemens and Juan Abreau to Houston.
Span currently ranks 12th in WAR among centerfielders and could help some teams. However, I don't know if they should expect any more than the Astros received for Bourn who has been a better player over the last two years.
Look at the trades over the last few summers. I don't see many #2-3 pitching prospects changing hands. Last year they were included in trades for Ubaldo Jimenez, Hunter Pence and Carlos Beltran.
Fortunately the Twins are in a good position with Span. Worst case, they keep him unless someone comes in a blows them away.