Such cost computation as is referred to in Section 13.08.180 and 13.08.190 shall be based upon a six-inch main, unless a larger main is necessary, in the judgment of the utility, to provide the proposed customer with the necessary services, including fire protection service; in which event the cost of such larger main shall be used in computing such cost. If, for the utility's future extension plans, a larger main than is necessary for the service required from the main extension involved is proposed to be installed, the difference in the cost between the larger pipe size installed and the pipe size required to provide the service, if any, shall be borne by the utility. Should a prospective customer or developer offer to construct at his sole cost and expense and to donate to the utility any transmission main extensions or distribution main extensions necessary to serve the proposed customer, as described by Section 13.08.200, and should the utility require, for its future extension plans, a larger main than is necessary, for the service required for the main extension involved, the utility may enter into a purchase agreement with the proposed donor to purchase the oversized main; provided, that the purchase price shall not be paid in whole or in part until such construction shall have been fully completed and accepted by the utility under the terms and conditions described in Section 13.08.200, as if such main were a fully donated main. The purchase price shall not exceed the difference in cost between the larger pipe size installed and the pipe size required to provide the service. Any costs to the utility referred to in this section may be determined from actual bids received by the utility or may be computed by applying the actual average cost, not reflecting unusual costs incident to special construction, experienced by the utility during the preceding twelve-month period, plus or minus any amount necessary to adjust for known cost increases or decreases, respectively. (Prior code § 33-32)