27-1-306. When replacement value to be allowed. The measure of damages in a case in which the cost of repairing a motor vehicle exceeds its value is the actual replacement value of the motor vehicle rather than its "book" value unless, after the damages arise, the parties agree to use the "book" value. "Book" value must be determined by referring to the most recent volume of the Mountain States Edition of the National Automobile Dealers Association (N.A.D.A.) Official Used Car Guide or the National Edition of N.A.D.A. Appraisal Guides Official Older Used Car Guide. Actual replacement value is the actual cash value of the motor vehicle immediately prior to the damage. "Book" value may be used to assist in determining the actual replacement value of the motor vehicle.