Barry Bonds' 700 HR ball sold for over $800,000 at an online auction that ended today.
http://sportsillustrated.cnn.com/2004/baseball/mlb/10/27/bonds.700.ap/index.html?cnn=yes
The seller, baseball fan Steve Williams, got the ball after a scramble in the bleachers at SBC Park on Sept. 17 -- a mad dash that may have enriched him but also landed him in court. Timothy Murphy has sued Williams, saying he should get the ball because he had it locked behind his knees before Williams swiped it.
EDIT:
Comic strip creator Todd McFarlane paid $3.2 million for Mark McGwire's 70th home run ball in 1999.