(a) A political subdivision that receives financial assistance may charge persons in an economically distressed area in which water supply and sewer services are furnished an amount for those services that is not less than the amount provided in the application for financial assistance.
(b) Except as provided by Subsection (c), the amount charged under Subsection (a) of this section may be equal to or less than the rates paid for water supply and sewer services by residents of the political subdivision.

(c) A political subdivision holding a certificate of convenience and necessity described by § 13.242, that extends service to an economically distressed area outside the boundaries of the political subdivision, may not charge the residents of the area rates that exceed the lesser of:
(1) the cost of providing service to the area; or
(2) the rates charged other residents of the political subdivision plus 15 percent.