I found this Philipp Schmitt et al. Study on the profit impact of customer referrals very instructive. Please note particularly:
“We studied 10,000 accounts in a large German bank over a period of three years, and found that customers obtained through referrals are both more loyal and more valuable than other customers. After controlling for such factors as age and gender, we calculated that referred customers are, on average, about 18% more likely than others to stay with the bank. We also projected that they generate 16% more in profits (amounting to €40 each). Thus, the bank earns a return of about 60% on its €25 referral reward.