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Based on data gathered from his Steam Spy analytics platform, programmer Sergey Galyonkin reports that people spent roughly $3.5 billion on full, paid Steam games -- not free-to-play games or downloadable content -- in 2015.
As always, that Steam Spy data comes with caveats. Galyonkin's report assumes all copies tracked were sold at their minimum advertised price. Also, not all of those Steam game sales actually came through Steam -- many developers sell Steam game keys via their websites, at retail or on digital storefronts like the Humble Store. Plus, Steam Spy only came online in April of last year.
Still, his report helps shed some light on the amount of money flowing through Valve's platform on a yearly basis, which is likely higher than the estimated $3.5 billion when you factor in money spent on downloadable content and microtransactions in games like Dota 2 & and Counter-Strike: Global Offensive -- spending that wasn't included in this report.
The rest of Galyonkin's report -- the promised follow-up to a prelim report last month pegging Grand Theft Auto V and Fallout 4 as the top-selling Steam games of 2015 -- is worth reading in full, as it also contains more details on how the figures were calculated and provides month-by-month estimated breakdowns of Steam game sales and revenue.