The pro-rate is simply based on the days of the month: You are charged based on the days you’ll be using the system, so the price will decrease linearly until day 28 of the month. There’s also a minimum amount that varies depending on currency.
Important: Keep in mind that the pro-rate only applies when your TaxiCaller account is on its ATH (All Time High) meaning that the next vehicle activation will place your account on the highest number of vehicles your account has ever been on.)