One needs to specify if this means "cheaper than a new construction fossil fuel plant" or "cheaper than operating an existing fossil fuel plant". Whether a CO2 tax is involved should also be specified. Things will really take off when existing plants can no longer compete (particularly in places like the US where overall demand is growing slowly, if at all). Of course, this will depress the price of the fuel (natural gas is in the doldrums in the US) which will delay things.