In a complaint to the Federal Energy Regulatory Commission, Madigan said prices produced by an auction last fall were 40 percent higher than prices in wholesale electricity markets, and approximately double the actual cost of generating electricity to serve ComEd and Ameren customers.
The complaint seeks modification of ComEd's and Ameren's contracts with the energy suppliers to reduce prices.
A study by Argonne National Lab and the University of Illinois shows that the marginal cost of generating electricity to serve ComEd and Ameren customers is less than $30/MWH over 90 percent of the year and less than $36/MWH over 95 percent of the year.
ComEd and Ameren customers are paying an average of over $70/MWH for electricity purchased through the auction.
"Ameren and ComEd customers should not have to pay twice the cost of generating the electricity needed to serve them," said Madigan.
A spokeswoman for ComEd's parent company, Exelon Corp., said the charges are false. Ameren Corp. had no immediate comment about the matter, Copley News Service reported.