I've heard that in most battles prior to the introduction of gunpowder weapons, the casualties were usually very low (around 5% even in long battles) prior to the moment when someone's formation was broken and the side stopped really fighting and just ran away (or was fighting without the benefits of proper formation if the side couldn't retreat), and that most people killed in battle were killed by the pursuing victorious force.
Is it true? Is it a matter of debate between historians? Is it basically true but exagerrated? Or is it considered a myth?