Petrol is only possibly a better example because all the damn oil companies have purchased rights to a few of the other possible ways to make fuel so no one can make them to try and help their monopoly-type setup
Economics is taught at GCSE in many places. I don't really think it gets *hugely* theoretical until A level etc though (I admit I can't really remember what I did in my GCSE course or in my AS levels but I'm sure it was fairly basic stuff at GCSE).
It was things like money supply affecting price - "if there's 3 chairs on an island and 3 people want to buy chairs, if they have £3 each then they'll be willing to pay less for a chair than if they had £10 each" and so on, to introduce basic concepts.
Really, subjects overlap quite a lot until it gets to about A-Level. Doing economics or business studies depends on if you want to go into a business or become some sort of adviser