Section 17B. In a city under Plan F the mayor and the members of the city council shall receive for their services such salary as the city council shall by ordinance determine, and they shall receive no other compensation from the city. The compensation for the mayor shall not exceed fifteen thousand dollars annually in cities having a population of less than fifty thousand or twenty thousand dollars annually in cities having a population of fifty thousand or more. The compensation for city councillors shall not exceed two thousand dollars annually in cities having a population of less than fifty thousand, or three thousand dollars annually in cities having a population of fifty thousand or more.