It would cost much more than $50 Billion to replace that generation capacity. Here are a couple options:
Coal currently produces a ballpark average of 200,000 Megawatts of power for the US continuously (based on the published annual generation totals). So we use that number to figure out how much of alternative generation sources you would need.
Lets start with Nucular since that is (relatively) simple with almost continuous output: Most recent power plan under construction in the US is Watts Bar 2 (look it up) that will produce 1180 megawatts at a cost of ~4.5Billion. To replace 200,000 megawatts you would need about 170 of those Watts Bar 2 plants at a total cost of approximately $763Billion in today's dollars! Wowser. Also note that there is a fuel shortage looming that makes spinning up new multi-billion $ reactors a potential waste of time and $. http://www.nytimes.com/2013/10...
Ok, lets move on to renewables. There are lots of options here...wind, solar, concentrated solar, tidal power, ect. The thing with non-hydro renewables is that most of them are cyclical or random like the weather. So you need huge storage capacity to go with them. Projects like the Bath County Pumped Storage Station are a great solution for this, but for 200,000 megawatts of renewables, you would need about 70 of those at a cost of $100Billion+. Thats just for the glorified "battery" and does not count the cost of the generation facilities themselves which would be hundreds of billions if not $1Trillion+.
Just keeping things in perspective:)