The annual amount to be paid to the authority by the authorizing county is to be ascertained by first determining the amount of ad valorem taxes collected for the county from within the city limits of each municipality served by the authority. The authorizing county shall pay to the authority each year out of its general funds an amount equal to five and one-half percent (5.5%) of the first eighteen million, one hundred eighty-one thousand, eight hundred nineteen dollars ($18,181,819) of ad valorem taxes collected within the city limits of all municipalities served by the authority plus and two and one-half percent (2.5%) of the ad valorem tax collected within the city limits of all municipalities served by the authority in excess of eighteen million, one hundred eighty-one thousand, eight hundred nineteen dollars ($18,181,819). In addition, the governing body of the authorizing county shall, at its sole discretion, make a determination prior to the end of each fiscal year as to the number of residents of the county residing in the unincorporated area thereof having access to services by the authority within reasonable walking distances of their residences, but the number shall not be less than in any preceding year, unless there is a reduction of routes serving the unincorporated area or the annexation of a portion of the unincorporated area served by the authority by a municipality. Such residents shall be deemed to be served by the authority and there shall be paid annually out of the general funds of the authorizing county a sum equal to one dollar ($1.00) per capita of the residents thus deemed to be served.