It is pretty unusual for a beginning diver to buy tanks. My thoughts are that tanks are expensive and unless you have a way to fill them it will not save you a trip to the dive shop. What does your shop charge for an air fill and what do they charge for a rental? Subtract the cost of the fill from the rental cost and divide the cost of buying a tank by that number. That will tell you how many dives it will take to break even on a tank purchase. Then factor in annual visual inspection and hydro every 5 years as additional costs of being a tank owner.