I'm not by any means a preservation expert, but a simple plastic ball-holder will go a long way to prevent aging by keeping the ball (and the ink) safe from both accidents and oils on fingers. Keeping the ball away from direct sunlight also makes a big difference.
As to the value, I really couldn't say. Even without Mantle, there are some great players there, and Yankees memorabilia is always popular. One factor that might keep the value down, though, is the lack of authentication of the signatures.