Having studied Environmental science the concept of 'externalities' is familiar to me, and many, but certainly it seems not to politicians, who don't consider them, or ignore them. Simply put the cost of burning coal for example is significantly higher than the cost of coal/transport/staff etc, The true cost includes healthcare and insurance costs for lives devastated or ended by the pollution by product of burning coal.
The bottom line is that renewables are NOT too expensive, they can pretty much already compete on what is a very non-level playing field. We have to stop subsidising fossil fuels, and start charging them to compensate for their externality impacts. Then renewables will be an exceptionally cheap option, even before we get the significant cost reductions we know are coming.
A 2011 study led by the former head of the Harvard Medical School Center for Health and the Global Environment (who is now deceased) found that burning coal for electricity cost the United States ~$500 billion a year in externalities — real costs not included in the price of electricity from coal (healthcare costs, premature death costs, etc.). What that worked out to was an additional 9–27 cents per kilowatt-hour.