In his 2005 fund letter, Nick Sleep mentions the Robustness Ratio.

The Robustness Ratio is a framework one can use to think about the size of a company’s moat.It is the amount of money a customer saves compared to the amount earning by shareholders.

This ratio is clearly appropriate for companies which aim to provide goods/ services to customers at the lowest possible price( eg. Costco, GEICO). For example, GEICO policyholders save $1bn on their policies when compared to the next cheapest carrier and GEICO itself earns $1bn on the issued policies.

The higher the ratio, the harder it will be for competitors to compete against the business. Costco saves the customer $5 for every $1 it retains, this clearly gives Costco a huge competitive advantage.