A rational market participant should be indifferent to buy an asset now or in time. Pricing of products should be accordingly, including commodities and financial instruments. Therefore, the forward or future price should equal the spot price plus the cost of carrying the underlying asset forward in time.
Price future = Price spot + cost of carry
The cost of carrying a security (like a bond or stock) forward in time are the cost of capital minus the financial compensation (like the coupon or dividend payment).
Unlike financial instruments, commodities do not face any financial compensation, but require physical handling, including storage. As a consequence, the cost of carrying a commodity forward in time are the sum of the cost of storage, the cost of insurance of the cost of capital.