Cal. Gov. Code § 51350
A county which provides services through its appropriate departments, boards, commissions, officers or employees, to any city pursuant to contract or as authorized by law, shall charge the city all those costs which are incurred in providing the services so contracted or authorized. A county shall not charge a city contracting for a particular service, either as a direct or an indirect overhead charge, any portion of those costs which are attributable to services made available to all portions of the county, as determined by resolution of the board of supervisors, or which are general overhead costs of operation of the county government. General overhead costs, for the purpose of this section, are those costs which a county would incur regardless of whether or not it provided a service under contract to a city.
Any determination of general overhead costs shall be subject to court review as to the reasonableness of such determination.
This section does not apply to a contract or agreement in effect on December 31, 1983, made by a county.
Ca. Gov. Code § 51350