From the Harvard Business Review Daily Stat:

An affluent customer who's willing to recommend his or her bank to friends is worth $9,500 more to the company than a similarly wealthy customer who spreads negative reports about it, according to The Ultimate Question 2.0, by Fred Reichheld with Rob Markey. Promoters of a business are usually less price sensitive than other customers, and they appreciate the overall quality they receive from the company; the opposite is true for detractors, who have a negative lifetime value, the authors say.