(a) As agreed by the retirement system and the governing body of a political subdivision that has an approved plan, the retirement system shall require the political subdivision to pay the subdivision's proportionate share of the state administrative expenses for this subchapter by:
(1) an annual fee for the political subdivision;
(2) an annual fee for each employee covered;
(3) an amount equal to a percentage of the contributions made to federal authorities; or
(4) any other equitable measure.
(b) A political subdivision may make payment for administrative expenses under this section from any available fund not otherwise dedicated.