This is a link to an August 15, 2018 Joule article: "A Process for Capturing CO2 from the Atmosphere." Thanks to Chad for tweeting this link.

For those who think a result like this has an anti-green message, let me say that, even if true, the headline number only implies an upper bound of $94 to $232 on what a carbon tax should be. A carbon tax of $90, if otherwise justified, would still be optimal. And a carbon tax of $90 would have huge effects in greening the economy.  

As Roger Bohn tweets, once the carbon dioxide is captured from the atmosphere, something would need to be done with it. If no better commercial use can be found for the carbon dioxide, the cost of long-term storage should be added to the $94 to $232 estimate.