The contract may provide that the association may sell or resell any gas which is delivered by its members, with or without taking title to the gas, and pay over to its members the resale price, after deducting all of the following:
(a)
Necessary selling, overhead, and other costs and expenses, including interest on preferred stock, not exceeding 8 percent per annum, and reserves for retiring the stock, if any.
(b)
Other proper reserves.
(c)
Interest not exceeding 8 percent per annum upon common stock.