How much would a power plant that generates electricity from gasified coal cost consumers and businesses?

That's the question Illinois legislators are asking as they decide whether to commit electricity users to an agreement that would have them purchasing electricity from the plant – to be built near Springfield by Omaha-based Tenaska Inc. – for the next 30 years.

Opponents of the project, among them Chicago-based electricity generator Exelon Corp., say Tenaska's big idea keeps getting more expensive. As natural gas gets cheaper, Tenaska's electricity from synthetically produced natural gas costs more: About $100 million more per year since a cost report was last commissioned in 2010.