I've blogged in the past about how much Ontario pays for gasoline. Ontario produces very little oil itself, so almost all gasoline is imported. Obviously with oil prices cratering, the amount Ontario overall pays for gasoline will go drastically down.
This Bloomberg article says that Canadian drivers will save $30 per week (around $1500 per year). That seems high to me. Assuming 5,000,000 million drivers in Ontario that means $7..5 billion is being saved (and that's in after tax dollars) per year with the cheaper prices.
A lot is being saved in any case. Ontario will lose out on some taxes as their 8% portion of the HST will decline with cheaper gas. However this money could well be spent on other items that HST is charged on so it is probably a wash for the government. Likely anyone selling goods and services that are purchased with disposable income are going to have more sales. Or Ontarians will put it towards debt reduction (which I doubt).
One issue for Ontario is that the banks headquartered here aren't likely to do well with lower oil prices and that means lower corporate income tax for Ontario, where corporate tax receipts have already been under expectations recently. The stock market is also crashing, with the TSX not much up year to date. That means less in taxes for capital gains. I'm not sure whether the big savings in gas counteracts these effects for the Ontario economy, although the lower dollar has to help exports and hurt imports (including tourism dollars spent in the US).