I scraped Stubhub.com's tickets for the October 2nd game at Fenway Park (vs. Toronto), which is the final game that the Red Sox will play in the regular season. I then compared them to the list prices for that game on MLB.com. The results aren't so much surprising as they are nauseating.
Excluding State Street Pavilion Club tickets, which are in the several-thousands range, the average price of a ticket to Sunday's game is $390, with a maximum price of $999. The average markup over list was 516%, with a maximum of 2705% for a standing ticket. There's also a bleacher ticket available for just 2337% over list, if you're interested.
If you're heading to the game on Sunday, I hope it's a good one - you sure paid for it!