How much would a signed baseball of rod carew be worth?

As I do not know the condition, nor the point in his career that he signed your ball, I cannot give a specific price.

However, for now, I can give you THIS LINK (http://www.google.com/products?hl=en&q=rod+carew+signed+baseball&um=1&ie=UTF-8&ei=K3JlSqfqOI7SsgOL85DeDg&sa=X&oi=product_result_group&ct=title&resnum=4), which contains many Rod Carew signed balls, to give you a rough estimate, depending on factors including those I listed above.