AC has a much higher fuel surcharge, so you have to compare the price after tax -- it might be very similar. But the whole idea of flying RJ vs. a larger plane is to shed the cheap fares and sell only the more expensive tickets. Also AA might be going after the connecting flights market for YYZ-BOS while AC is primarily O/D (does UA have any trans-Atlantic flights ex-BOS?). So AA doesn't want to sell YYZ-BOS as much as they want to sell YYZ-BOS-DUB, etc.