Definition
Customer retention cost is what a business spends to keep its existing customers: customer success, support, account management, loyalty programmes and retention campaigns.
Formula
Retention cost per customer = retention spending in a period ÷ customers retained in that period.
Using it
Compare it with acquisition cost and with the revenue retained customers bring. Spending that keeps a customer for another year is usually cheaper than replacing them. Product signals help spend it well, on the accounts whose usage shows risk rather than on everyone equally.