Since the advent of RS gold grand exchange, objects can be exchanged for money without difficulty, and can be effectively considered the same thing. Consider that (hypothetically) you need 100k cannonballs in order to slay, and you are able to purchase 200k with a price of 250gp per. If you buy 25000k 100k cannonballs up to 300gp, you'll be left with 30000k that you can use to convert into cash...