In game terms, Cannons are the Modern Armor of the day. Collateral damage, huge combat capabilities, with the addition of bombard.
Cannons should be weaker! 8 would be nice, so that they act like "catrebs."
As for the Musket and Longbow costs, I think the game devs just wanted to say that guns are better than bows, no matter how old.
I guess even the old "pan-lock" handgun (the one that looks like a WWII Nazi grenade) with its barely-trained user is more than a match against a well-crafted longbow in the hands of an expert woodsman. Or against a well-crafted crossbow with much greater range and accuracy in the hands of an equally untrained soldier.