The shopkeeper says "It's $25 for the rat and $100 for its secret".
The chap decides to just pay the $25 and leaves the shop feeling contented. As he walks down the street, he sees a rat following him and thinks it a little odd. He walks a little further and there are a dozen rats behind him. Eventually he reaches a bridge, by which time all the rats in the whole town are following him.
He thinks "I've had enough of this - it must be that golden rat" and he throws it into the river. To his amazement, all the rats jump into the river after it and drown.
The next day he goes back to the shop and the shopkeeper greets him: "Nice to see you again. I bet you've come to learn the rat's secret".
He replies "No, I've come to buy that golden banjo".