McFarlane purchased a ton of "significant" home run balls and is on record as saying they weren't bought as investments, but rather because he wanted them. Scott G's point about publicity is also a very good one. There has been a 30 for 30 made about the ball, he got an AMAZING amount of press at the time etc... you could add up the price of all the balls he bought and then figure out what the hundreds of print articles and TV air time those purchases garnered would have cost as advertising space/time and he came out WAY ahead.
A little perspective:
http://content.time.com/time/arts/ar...650788,00.html