In the event two (2) or more municipalities jointly enter into an agreement or agreements, the proportion of the moneys to be paid by each of the municipalities shall be determined and stated in the agreement based, from time to time, upon the percentage of population of each municipality within the group in relation to the entire population to be served by the facilities, together with other factors that may be equitable; as, but not limited to, amounts of industrial and commercial wastes in any of the municipalities, and allowance to that municipality in which the disposal facilities may be located.
R.I. Gen. Laws § 45-34-5