(This is about Toronto's greenbelt, not Ottawa's. Proposing to build on it is a Big Deal.)
Ottawa's greenbelt is about 200 square kilometres; the GTA's is about 7,200. They both hem in urban sprawl (somewhat), and one of the predictable consequences of that is higher land prices within. Ford proposes to reduce prices by allowing more building on protected land.
Adding more single-family houses at the far outskirts of a metro area also has predictable consequences for traffic, infrastructure, and service costs.
The more so if your only acceptable form of public transit for Toronto is subways, of course, which single-family homes don't really support.
Here is a map of population density in Toronto proper in 2011. The GTA greenbelt is WAY outside the boundaries shown here. There's a lot of room for more housing in Toronto — just not single-family housing. Any more than in Vancouver or New York. 
If single-family housing is all we are willing to accept, however, it will still be expensive and come at the cost of hours on clogged freeways and public services that require massive permanent subsidies to make up for their inefficient population distributions.
The basic problem is that sparse populations don't pay the taxes it takes to support robust public services. Low-density areas are expensive to police, educate, supply with libraries, transit and pools.
You need long pipes that deliver water and take sewage from relatively few homes. You need more fire stations and paramedic posts, all to cover geography rather than population.
That can be OK if people are willing to pay very high taxes.
Most people are not willing to pay very high taxes.
Most people are not willing to pay very high taxes.
There is basically nothing we can do about the traffic.
In theory, we could build self-contained communities where a family can live, work, shop and play. In practice, parents will probably work in different parts of the metro area. Maybe one kid goes to school close to home, while another spends two hours going downtown for college.
I grew up in Mississauga. My dad worked downtown. Mom worked in Brampton. Grandparents lived in North York. I eventually got a summer job in Markham.
Driving, driving, driving, driving. Transit was never a serious option for any of these trips. Still wouldn't be, I don't think. (I did take a city bus to high school.)
Here is a map of the current GTA greenbelt plan. If you feel like the solution to expensive housing in Toronto is more single-family homes beyond Caledon and Ajax, fill your boots.
