I buy a rowboat to use in the lake. I have to buy oars to row it. I have to buy a permit to use it on the lake. I have to buy life preservers for safety. I have to buy a trailer to drive it to the lake. This is the first layer of cost.
I decide to put an electric motor on my boat. I have to buy a motor. I have to buy a battery for my motor. I have to buy a charger for my battery. I have to pay for electricity to use my charger. This is the second layer of cost.
So putting a motor on my rowboat gives me an added layer of cost for my rowboat, since I have to spend money on several different things to do that one task.