Whenever the cost of purchased power or energy changes, the effect of the change on the cost of service shall be determined and the Area Director shall adjust the power rates accordingly. Rate adjustments due to the change in cost of purchased power or energy shall become effective upon the unilateral action of the Area Director and shall remain in effect until amended by the Area Director pursuant to this section. A notice of the rate adjustment, the basis for the adjustment, the rate schedule(s) shall be published and posted in the same manner as described in Sec. 175.12(c) of this part. The Area Director's decision shall be final for the Department of the Interior.