What you buy it for is directly related to what you can sell it for. For example Cu might trade for 3.50 in the financial markets, so that is what it is worth. You aren't trading the contracts so you cannot get 3.50 for it. You might be able to find a broker who sells to a foundry. The broker might get 3.50 or close, but he is going to give you a little less. The difference is his profit. You must determine what you can do to process the material and how much it costs to do so and or to store it, including risk. Deduct that from the price you can get from the broker, and that or less is what you can pay for it. This is the standard business model from my experience.