American Airlines and Air Canada have round-trip flights from Dallas (DFW) to Quebec City (YQB) for $264-$281 with a connection. Flights August to October. Very few weekends available.
To get the lowest price ($264) requires booking each leg of the trip as a separate one-way. The reason it’s cheaper is that you’re avoiding a few airport and customs taxes. Understand that you may have to pay those taxes at customs anyway.
A Carry-on bag and also a personal item is allowed for free on American. You can avoid paying for seat selection by accepting the randomly assigned seat at the gate.