If you want the cheapest method then you should get a earth staff, buy 2,334 body runes and 4,668 water runes. Then get your best melee armor and go to Varrock Castle and cast curse on the caged zammy mage under the stairs (you want the lowest negitive mage bonus possible so that you don't actually hit him it just splashes, so you can keep cursing him). This should get you to lvl 50 mage, it should only take you about two hours. It can be very tedious, but I think it's well worth all of the clicks it takes.