I don't understand how the conversion from FLOPs to dollars is done. A footnote says that "These $ figures come with substantial error bars, but we believe they

I don't understand how the conversion from FLOPs to dollars is done. A footnote says that "These $ figures come with substantial error bars, but we believe they are in the right ballpark." Where are the estimates themselves coming from? Cost of cloud computing? Cooling and power consumption estimates? I feel the article would benefit from adding a sentence fragment in the abstract or introduction that specifies this as I have no idea what I'm looking at right now.