Mont. Code Ann. § 27-1-306

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 used car value and pricing information published by an organization nationally recognized for providing information related to used car value and pricing. 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.