I just did a quick search of eBay via GetItNext to check and see what the final home game tickets at Yankee Stadium are selling for. Currently the range for the last NY Yankees home game via a Buy It Now is $384 for a ticket all the way up to $8000 for a pair for the 9/21 home game. A bit too rich for my blood and I also have no desire to go.
I've been to the House that Ruth built 100's of times and said my goodbyes earlier in the season. My fondest memory is sitting in the left field bleachers watching them win the series against the Atlanta Braves in 1996.
If you wish to follow ticket prices via traditional auctions check out this link to NY Yankees last home game auctions.