In 2011, I wrote a piece for Scientific American on the exponential price decline in solar power.
I haven't had a chance to fully update that piece, but two quick notes. First, the price decline in solar cost per watt has, if anything, accelerated since then.
The above is on a log scale. The Y-axis is price per watt of solar modules. And you can see that since 2010, prices have plunged. Over the last 30 years, in total, solar module prices have dropped by a stunning 95%. You can now buy literally 20x as much wattage of solar power for a dollar as you could when Ronald Reagan started his presidency. And the trend shows every sign of continuing.
Second, since many have asked about storage costs, I'd note that there is a long term exponential decline in the cost of energy storage as well. Energy storage is still far too expensive to be used to store substantial amounts of wind or solar energy, but the price decline, if anything, is steeper than the price decline in solar power itself.
Here's the original piece on the exponential decline in the price of solar power.