David Ortiz autograph?

i couple of months ago i was at yankee stadium when they were playing the red sox. i was standing around at BP and David Ortiz turned around and threw the ball that he was warming up with me to me. a couple of minutes later i wandered to the other side of the park with my dad. he was taking BP in the cage. when he walked out of the cage he walked to the seats right by the yankee dugout where a bunch of kids were standing and he started signing autographs. i ran down and got his autograph. the only problem is, is that im a huge yankee fan, so natrually i really dont like him. how much do you think his autograph is worth, seeing that i dont have a certificate of authenticy. id like to buy someones on the yankees. maybe arod. how much do you think?