Basically, you either train with extreme magic potions or overloads (at current prices overloads need over a 20m/h income to justify training with them). To turn 1k magic potion into an overload, you spend 73m, earning you 1,950k xp. That xp would cost you 54.5m with extreme magics. So each overload actually costs you about 6.2k per dose, or about 74k an hour. Anyway they aren't free, especially if you don't care about herblore xp (usually if player is 96+ or 99).