Air Canada has a sale going on: fly to the US 48 for $159 one-way or twice that for round trip from Toronto, Ottawa, and Montreal. Sale ends May 1, and travel is through June 30. Of course, once you pay all those taxes, a trip from, say, Montreal to Dallas will end up costing $445 RT. That includes a Canada Goods and Services tax of $18 and a Canada Airport Improvement fee of $20.
We compared that route on Travelocity.ca, same dates, and found that you'd pay about $25 more there.