2. Customer Retention Cost (CRC): How much it costs your company to retain and market to existing customers/donors. Same as CAC, but collect expenses and results only for efforts targeted to existing customers. Remember, we are measuring total customers who responded not the total number of transactions. That way you can see how many people responded not orders. From a stats perspective, once a new customer buys for the second time they move into the retention pool.

Customer Retention Cost Calculation:
Total cost of retention campaigns / Total # retention customer responses

See all 6 tips here