I agree with several others here that it would depend on the player and the milestone. With a Brewers player, I would definitely test the waters to see what I can get but in the end, he is getting that ball. Any other player I would really depend on the milestone. If it is just some opposing teams rookie's first career HR, an autograph from a player or tickets to a game would be all I need. A milestone like Jeter's 3000th hit and I would probably need to see a nicely sized check. I am curious if it has ever been discussed (or even possible) among major league teams to put in the terms and conditions of a ticket purchase that the teams reserve the right to get back any ball without compensation. I'm sure most teams would give compensation anyway but this would make it much easier for them to prevent people from holding out for huge money.