Revenue per User (RPU) measures a company's profitability on a per-user basis. You simply take overall revenue and divide it the total number of users or customers.
RPU = Total Revenue / Total Users (Customers)
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.