Because Co-generation is more efficient you buy up to 30% less energy (Electricity and Natural Gas).
In California the average cost per KWH is 12 cents and the average cost for a Therm of Natural Gas is 70 cents.
Co-generation delivers a kwh for about 7 cents and for every KWH delivered there is a 2 cent reduction in Natural Gas costs because the waste heat is captured and used to heat water, therefore reducing the amount of NG burned in boilers.