No, you only pay in advance when you take the bus. When you take a cab/taxi, you set the rate in advance, but only pay at the end (when the currency is less valuable).
"During the Brazilian inflation of the earl 1990s, for instance, supermarket workers reportedly spent half of their time replacing old price stickers with new ones." -- Essentials of Economics
But 100 pesos at time T (bus) is worth more than 100 pesos at time T + interval (taxi)