If you take a look at the comment that I was replying to, you will have your answer for the first question

I didn't include coal plants because we weren't talking about coal plants.

So I should just ignore the first paragraph of your comment, where you introduce your own context for the rest of the comment and mention replacing coal, oil, and nuclear plants? The person you replied to suggested a solar panel array. You stated you wanted to "play around with the thought of powering all of America with renewables" but then excluded all forms of renewables from your analysis except for two very specific options.

Let's play around with the thought of powering all of America with renewables. America’s coal, gas, petroleum and nuclear plants generate a combined baseload power of 405 GWavg, or “gigawatts average.” (Remember, a gigawatt is a thousand megawatts.) Let’s replace all of them with a 50 / 50 mix of onshore wind and CSP (solar)

Solar is not one of the "most expensive" forms of renewable.

Who said it was? I said you chose the most expensive way of implementing it with concentrated solar power, a giant array of mirrors that focus light into a central tower. What about regular solar panel installations that don't require a gigantic central facility hundreds of miles from population centers?

Your analysis reads like pure misinformation intent on leading people to believe our only two choices are to stick with coal, oil, and nuclear or pay multiples of our nation's GDP and use several years worth of resources like steel in order to go the "renewable" route.