The Cost Performance Indicator is the same thing at the Cost Performance Index, and thus it is calculated the following way:
CPI = EV / PV (where EV stands for Earned Value and PV stands for Planned Value)
Another formula for CPI is:
CPI = BCWP / ACWP (BCWP is the Budgeted Cost of Work Performed, and ACWP is the Actual Cost of Work Performed)
By the way, the term Cost Performance Index is used (much) more often the the term Cost Performance Indicator, but both are valid.