Bonds issued under Section 10 of this Act shall be payable solely from the revenue derived from the operation or leasing of the airport, landing field, and facilities or appurtenances thereof. These bonds shall not, in any event, constitute an indebtedness of the county, within the meaning of any constitutional or statutory limitation. Each bond shall plainly state on its face that it has been issued under the provisions of this Act and that it does not constitute an indebtedness of the county within any constitutional or statutory limitation.
     These bonds shall be sold in such manner and upon such terms as the county board shall determine. If the bonds are issued to bear interest at the rate of six per cent annually, they shall be sold for not less than par and accrued interest. If the bonds are issued to bear interest at a rate of less than six per cent annually, the minimum price at which they may be sold shall be such that the interest cost of the county of the proceeds of the bonds shall not exceed six per cent annually, computed to maturity, according to the standard table of bond values.