Upstart Porter Airlines is now flying out of Halifax to Ottawa and Montreal with connections to Toronto. Fares to Montreal appear to start at about $79, with Ottawa around $124 and Toronto starting around $114. Taxes and fees add about another $50.
Porter is flying the Bombardier Q400 turboprop: Halifax to Montreal takes 1:45, vs. 1:30 on an Air Canada jet. From the looks of it, Air Canada is matching fares at least some of the time — both Porter and Air Canada were charging the $79 one way fare for Halifax to Montreal in mid-July when I checked tonight.
Of course only Porter can boast a Tyler Brûlé-designed in-flight magazine (the first issue as a PDF).