Cost performance index (CPI) provides a direct measure of how well you are doing with regard to cost. It is defined by:
CPI = EV / AC
where EV is earned value and AC is actual cost.
A CPI of less than 1 indicates a cost overrun. A CPI of greater than 1 indicates a cost underrun.
Budget at completion (BAC) = $1000
% of work completed = 60%
Earned value (EV) = $1000 x 60% = $600
Actual cost (AC) = $750
Cost performance index (CPI) = $600 / $750 = 0.8