I think tolling the existing roads would put a huge disincentive on travel within the GTA. Many people rely on the highways to get to/from work or to conduct business within the GTA. Putting tolls on these roads would be completely irresponsible given that our transit system is incapable of offering an alternative.
Cities like London, New York, Stockholm are freely able to add tolls because their transit system is superior and provided a more than adequate alternative to driving.
In Toronto, you would have a revolt if the 401, DVP, Gardiner were tolled. What are people supposed to take instead?
GO transit is full, and only useful to Union station. TTC subway is full in rush hour, Yonge line is beyond capacity. Spadina has some capacity but not that much. B-D line is very full as well. However, the subway is not for everyone, and again is focused only on downtown.
Buses and streetcars are too slow and are inconvenient. Also, with highways tolled, arterials would be more congested, making these options even more inconvenient and slower.
My point is this. The GTA should not even consider levying one penny of tolls on the highways until a proper transit system is built. Governments should take up debts to fund projects and tolls should be used to help pay for them. When we have all-day bi-directional GO service in a web around the city, a much better subway and LRT system, then we can toll the highways.
I would prefer that governments raise gas taxes in the GTA to help pay for transit improvements rather than charge tolls on highways. The main problem is that money is not used properly by the government. It is colossally wasted on political pet projects that don't to much for the system. There are more than enough tax revenue to go around. The problem is colossal waste by all levels of governments, caused by slow and incapable processes that are constantly delayed.
We have not made any real transit improvements since the 1970's people. The region's population has doubled in size, yet transit capacity is only marginally higher.
Built it first, then toll it. That's the best approach IMO.