According to Wiki -
http://en.wikipedia.org/wiki/Ashes_series - the urn contains the bails of the infamous defeat in 1882.
However - the prize is actually a waterford Crystal trophy, and the Urn itself is in the museum at lords, and has been for some time..
So depending on the wording of your bet - you may have a get-out clause, as the prize is neither ball nor bails, but a crystal trophy.