The board may, in its discretion, accept or reject the said petition. If it deems it for the best interest of the district that said petition be granted, it shall enter an order granting the said petition and from and after such order, the said petitioner shall be deemed to have agreed to the acquiring of water service under the terms set forth in said petition and order. Such order shall provide for payment on the basis of rate per acre-foot of water service contracted for said lands within the district; Provided, that the board may divide the district into units and fix a different rate per acre-foot of water in the respective units; and provided further, that such rates shall be equitable although not necessarily equal or uniform for like classes of services throughout the district.