Can you replace the question mark with a number?

An elderly couple visits their grown-up grandson one night. While in the bathroom, Grandpa discovers a bottle of Viagra in his grandson's medicine cupboard.

"I don't think you should take one of those," says the grandson when his grandpa asks him about them: "They're pretty expensive."

"How much?" asks the old timer.

"$20 a pill," replies the grandson.

"I'd still like to try one," says the old man: "Before we go in the morning I'll leave the money under the pillow in the guest room."

The next day the grandson goes into the guest room, and lifts the pillow to find $120. Puzzled, he calls his grandpa. "Grandpa, I told you the pills were $20 each!" he says.

"I know," says the old man: "The extra $100 is from your grandma!"