Duncan also said the Ontario Power Authority paid Greenfield South, developers of the cancelled plant, another $10 million as a sign of good faith while trying to persuade the company to halt construction.
"There are no other costs that I'm aware of," Duncan told reporters.
Energy Minister Chris Bentley said last week it would cost $180 million in penalties for the move to cancel the plant just days before last fall's election, a decision he said had been made by the Liberal election campaign.
Duncan said Monday the decision to scrap the plant was made by Premier Dalton McGuinty.
"It was in fact a government decision and ultimately the person who made that decision was the premier," he said.
McGuinty has not been available publicly to comment on the cost of cancelling the project since it was announced.
The opposition parties complained the Liberals had given out two different figures and said the auditor general should be called in to determine the actual cost of cancelling the Mississauga plant and another one in neighbouring Oakville.
"The public have been dinged for $190 million and they deserve to know if there are more expenses than what’s been revealed so far," said NDP energy critic Peter Tabuns in an interview.
Tabuns wrote an open letter to auditor general Jim McCarter Monday saying "there is much uncertainty about the full cost" of scrapping the plant, which was well into its construction phase when the Liberal campaign cancelled the project.
"Government documents and recent reports indicate that the cost may be significantly higher" (than $180 million), he wrote.
"Ontarians deserve to know the full cost of the government’s last-minute decision to cancel and relocate the Mississauga plant, and whether or not the government exercised due diligence in minimizing the costs of the contract change."
The Progressive Conservatives said they are "very concerned the $180 million is just the tip of the iceberg" when it comes to the costs of cancelling" the two plants.
"What I worry about is that we’ve only seen the beginning of these costs," said Opposition Leader Tim Hudak.
"And secondly, we need answers on Oakville," he said. "Oakville was a larger plant (and) the cost there is going to be much higher."
The government is still negotiating with the developer of the gas-fired generating station in Oakville that the Liberals cancelled after local residents brought in high-profile activist Erin Brockovich to speak against it.
The Ontario Power Authority said Monday it did not offer any extra fees like the $10 million given to Greenfield South Power because construction never actually started on the Oakville power plant.
Construction on the Mississauga plant continued for weeks after the Oct. 6 election, but the decision to cancel it helped save five Liberal seats in the area.
The opposition parties called the decision to cancel the plant and relocate it several hundred kilometres away in the Sarnia area "a Liberal seat-saver program" and complained the government was not releasing all the relevant documents.
"The auditor digs in and finds out what’s actually gone on and he brings the facts and numbers to light," said Tabuns.
"I think he will have a better chance at getting to the documents that are there and, frankly, following up on paper trails that may no be apparent to those who do not do accounting all the time."
Hudak said the political decisions by the Liberals to cancel planned electrical generating stations that they originally said were needed to meet demand will turn off potential investors at a time when Ontario is overhauling and modernizing it's power system.
The Liberals' generous subsidies for wind and solar power in addition to their political interference in building new gas-fired plants are hurting Ontario, he added.
"The problem here is that we have turned energy from a major strength for attracting jobs to our province into a major detriment," said Hudak.
"And stories like this are going to undermine the confidence of investors in our province."
The Liberal government plans to turn off the last of Ontario's coal-fired electrical generating stations by 2014, seven years later than originally promised.