The father wants to show his son how to gamble and how to get the best deal and the son want the expensive pokemon cards.
The dad is telling his son how to get the best price. He tells him to go give a 10 dollar bill to the kid and then he will get the book for a good price.
The dad was angry that his son let that kid buy the pokemon cards for super cheap, but in the end the kid went home with what he wanted.