In this case, you are making an investment that really should have nothing to do with the number of cballs you need for RuneScape gold slaying. It is like how folks think that mining ore, banking it, then smithing it's coaching 2 skills at once with better efficiency - unless doing both together somehow saves more money and time than doing both separately (e.g. if there is a furnace on the road into the bank that saves you the time of having to run to a furnace after banking), they need to be treated as two individual events. Since the advent of the grand market, things can be exchanged for money easily, and may efficiently be considered the same thing. Say that (hypothetically) you need 100k cannonballs for slayer and can manage 200k, at 250gp each. Likewise, in the event the cannonballs rather fall to 200gp every day, you will effectively have 20000k, which you may convert to money or use. Doubling the investment simply doubles the return, if it be positive or negative. If you believe that cannonballs increases, then invest as much money as you want to them: the greater the investment, the greater the potential gain on whatever you do not end up using. If you wind up purchasing fewer cannonballs than you require, then if they rise you will need to pay for the rest at a higher cost. Basically, if you believe that cannonballs will grow, then invest in them regardless of how many you believe you will need for OSRS Gold For Sale slaying. If you're unsure whether or not they will rise, keep your cash as money and buy cannonballs as you go rather than stockpiling.