I guess the joke is that while you're in the taxi, pesos goes down, but the cab driver didn't have the time to adjust his pricing according to the new (lower) v

I guess the joke is that while you're in the taxi, pesos goes down, but the cab driver didn't have the time to adjust his pricing according to the new (lower) value of the pesos. Therefore he'll charge you the old price which is lower than what it should be by the end of the ride.