The purpose of this work is to investigate the variation of fuel and electricity's cost ratio effect on the optimal running cost operation strategy of a typical CHP system simulated with partial load models and meeting a hotel's demand profile. Different cost ratios were used representing cost increases in fuel and in electricity higher than 30% of the reference ones. The effect of the ratio variation over the economy done using CHP compared with the conventional system, as well in the fuel-energy saving ratio (FESR) and system's exergetic efficiency are presented. The economy, in running cost, reached 48% with a electricity cost 30% higher than the original condition, decreasing near 18% when the fuel cost was raised in the same rate. In the other hand, opposite effect was observed in the FESR, decreasing with the electricity cost increase, being around 2.25 times higher when the fuel cost increased and presenting more sensitiveness to the latter than to the former. When is possible to sell surplus, the system's exergetic efficiency was affected in 2% with a electricity cost increase of 30%. Further, the results showed that there is a stability range in the fuel-electricity cost ratio and that such range changed according the possibility of selling or not electricity surplus.