In determining the value of any public improvement for the purposes set forth in § 15.2-3211 the special court shall take into consideration the original cost thereof less depreciation, reproduction cost at the time of annexation less depreciation, as well as present value. The city or town shall receive credit, upon a basis to be determined by the court, for any sums it may have contributed to such public improvement and may in the discretion of the court be allowed credit for any portion of the cost thereof contributed by any federal, state or other agency and not borne by the county. When such improvements consist of a school financed in part from county funds and in part from a state grant, the city or town shall receive such credit only upon that portion of the cost paid for by the state grant and only then upon the ratio that children residing in the area annexed and enrolled in such school therein bears to the total attendance of school children in the county. The governing body of the county or any town therein, portions of which are proposed to be annexed, shall not between the entry of the decree of annexation and the date when the same becomes effective, make or contract for any permanent public improvements, to be paid for by the city or town seeking annexation, without the consent of the corporate authorities of the city or town and the supervision of the official thereof charged with making similar public improvements within the city or town.