You could sell numbered prizes to the winners, like t-shirts and other marked swag announcing how great the winner did. You might link the cost to the buyers p

You could sell numbered prizes to the winners, like t-shirts and other marked swag announcing how great the winner did. You might link the cost to the buyers position in the winner's list. I know one t-shirt company sells limited edition t shirts and charges a dollar per edition number (e.g. $20 for #20, $1 for #1).