It really depends on what a Yankee fan would want for it. There isn't much of a market outside of New York for him since he is all Yankees.
I checked a few sites, one had one for sale for around $450, but they are a huge company and they always overprice their stuff. For example, they sell Tom Arnold autographed baseballs for $89, and a Pat Borders baseball for $159!!!
It also says on that site that he rarely signs.
[list]Classic Jerry Colmanisms
[*]"Montreal leads Atlanta by three, 5-1."
[*]"They throw Winfield out at second, but he's safe."
[*]"Johnny Grubb slides into second with a standup double."[/list:u]