There were two plants that were cancelled. The auditor general reports are there:
Mississauga PDF (
http://www.auditor.on.ca/en/reports_en/mississaugapower_en.pdf)
Oakville PDF (
http://www.auditor.on.ca/en/reports_en/oakville_en.pdf)
They are quite readable and most of the information is contained in summaries at the beginning.
The gas plants were planned in 2004 to replace coal plant generating capacity. They were bitterly fought by local interests, including local municipalities, for years. They were cancelled in 2011. The two plants are still being built, but the Mississauga one in Sarnia and the Oakville one in Napanee.
There are four kinds of costs.
The cost of work done
The cost of equipment, parts, and supplies already purchased
The cost of getting out of financing agreements
The long term difference between what it might have cost to have the plants in their planned locations versus their new locations
$115 million was paid for work done. "Sunk costs".
$275 million was paid for equipment and parts. It isn't clear to me what will become of this equipment and parts. If they might get used, then this isn't really wasted money. If they aren't used or sold, then it is wasted money. Some of these parts seem to have been counted towards
$150 million was paid to the financiers of the Mississauga plant, who had it in their contract with the contractor that if the agreement was cancelled then the financiers would still get 14% interest for 8 years on amount they would have lent ($263 million). It turns out only $59 million of the loan was ever taken out before the cancellation happened.
The rest of the costs are nowhere near being paid yet, and most will be paid over a long period of time, and these are the additional costs for the policy decision to locate the plants in Sarnia and Napanee. It will cost more to get natural gas to Napanee, and it will cost more to get electricity from the two plants to where it will be consumed. Some other infrastructure things will have to happen for the Napanee plant. The actual cost of the electricity from these plants will be quite a bit lower, so this is counted as a saving in the reports.
So you have the sunk costs (actual work done and financial penalties) and the hypothetical future additional costs for the policy decision to have the plants elsewhere.
A good comparison is with the 2013 decision to change the Scarborough LRT to a subway. All the money spent on the LRT up to then is gone ($80 million+), and there will perhaps be some other sunk costs too. Then there is the future cost of the policy decision to build a subway instead (couple of billion more), and whatever difference there might be to actually operate a subway instead. If you add up all of those costs, you would end up with a comparable amount to what the costs are of the gas plant cancellations.