Reasonable ways of producing “free” energy

I know there's a lot of talk on the internet about "how to produce the free energy", "how to capture it" or simply how to lower or eliminate your monthly bills (electricity, gas, heating). In fact, the energy harvested here, on this site, doesn't come for free. It simply exists in space and time and we just find ways of collecting it with some price. What could be such price? What could be the price of denying that we all have to pay something one day and our children will breath a cleaner air than we do? What could be the "reasonable" price?