The statement "cheap energy" is really hard to quantify (like 'largest integer') you can say "Gee Coal is really Cheap and there is tons and tons of it!" Except that coal smoke is killing people, contaminating large swaths of land etc. But at $100 a barrel you can gasify coal and make clean gas power plants cost effectively, but what is the $100 really mean? Its local currency (US Dollars) in a world economy. At a high enough price you can build nuclear plants and fast breeder reactors that eat all their own fuel. And use that energy to make long chain hydrocarbons (aka oil) out of CO2 and electrolisys of water [1]. Politically nonviable but that too is a system, what do people vote for energy and synthesize oil for plastics or a degenerating quality of life? There is some thoughts that you can run F-T reactors using concentrated solar in the desert.

But to you point of 'running out of cheap energy' what does that mean if the economy has adjusted to the cost of that energy? Look at electric cars as a prime example, sure they are expensive today but what happens 10 years from now when they are everywhere? Now $200/barrel oil, converted into electricity at a fossil fuel plant is give you the same miles per $ as burning it as gasoline used to do in your internal combustion engine.

Constant adaptation by the system to the constraints applied to it.

[1] http://en.wikipedia.org/wiki/Fischer-Tropsch