Cost to Serve (by customer or segment) is a measure that helps to understand the profitability of a customer account (or segment) by assigning costs of serving this customer based on the level of activity and overhead costs incurred by this customer.
This indicator will be included in my book: Key Performance Indicators - the 100 measures every manager needs to know, which contains an in-depth description of this KPI, as well as practical advice on data collection, calculations, target setting, and actual usage.