Any securities herein authorized, except for warrants not issued for cash, and except for temporary bonds issued pending preparation of definitive bond or bonds, shall be sold at public or private sale for not less than the principal amount thereof and accrued interest, or at the board’s option, below par at a discount not exceeding five percent of the principal amount thereof and at a price which will not result in a net interest cost to the authority of more than six percent per annum computed to maturity according to standard tables of bond values.