Q: How much did Santa pay for his sleigh?
A: Nothing.  It was on the house!

Explanation:  If something is “on the house” it means it is free.  Often you will hear the expression in a movie, “Drinks are on the house!”  That means that the bartender or the owner of the restaurant will pay for the beverages.  In this case the sleigh is literally on the house.  That’s what makes the joke funny.