Said bonds shall be negotiated at not less than par as other bonds of said cities are negotiated, and the proceeds thereof applied solely to the payment of the cost of said improvement. The contract price of the same and interest thereon to the first day, when interest thereon is payable, shall be taken as the cost of said improvement, to be assessed on the property benefited, according to existing laws in each of said cities.
53 P.S. § 1804