I'm of two minds on this.
On one hand, having recently had to spend more time in Durham region, I can appreciate why an airport there would be a welcome alternative to Pearson. If you add up Durham region, Markham, and Scarborough you probably have a fairly decent sized passenger-shed which could justify some regional flights.
I'm more sceptical that Pearson needs some kind of relief airport. Given the terminal expansions and amount of runways they have, I don't understand why the GTAA feels it's going to run out of space. These claims seem like either gross optimism about future air traffic growth or empire building by the GTAA. Pearson isn't even fully built out!
Partially it depends on exactly what is planned for Pickering. A small-ish, regional airport need not be a Mirabel scale endeavour. Something on the scale of what exists at YTZ now but with a larger runway could be appropriate. But given how remote the area is, the project would need several major highway expansions and possible a (relatively) high speed link downtown, which would definitely drive up costs.