In Rotenberg v. Richards, 899 P.2d 365 (Colo. App. 1995), the Court found that an amount is either liquidated or determinable for purposes of 13-80-103.5(1)(a) if an agreement sets forth a method for determining the amount due, regardless of the need to refer to facts external to the agreement. There, the court held that the statute applied to an action to enforce a retainer agreement expressly providing for payment of $ 100 per hour for legal services, despite the need for external evidence to establish the number of hours worked.