Valve's PC network Steam generated close to $3 billion in total game sales during 2015, according to independent financial data.

Of that, the top twenty best-selling games are estimated to have brought in more than $650 million. Grand Theft Auto 5 singlehandedly sold about 3.8 million units, equating to $160 million, while Fallout 4 sold some 2.7 million units and amassed $122.5 million.

These estimates come from an independent marketplace analysis by Steam Spy owner Sergey Galyonkin, who in a blog post explains that his estimates are not completely accurate.

Galyonkin uses a web API to track the sales data of more than 3,000 Steam games that were released in 2015, and calculates revenue by using each game's lowest advertised price. That would suggest these estimates are conservative, and the final figures could be higher.

According to the Steam Spy data, Konami's latest blockbuster Metal Gear Solid 5 is believed to have made $41 million, while GameSpot's Game of the Year Witcher 3 brought in $38 million. Breakthrough indie hit Rocket League is estimated to have made more than $30 million.

Steam's 20 Best Selling PC Games in 2015

Grand Theft Auto 5 Fallout 4 Metal Gear Solid 5 Counter-Strike: Global Offensive The Witcher 3: Wild Hunt Rocket League Call of Duty: Black Ops 3 Mad Max Mortal Kombat X Elite Dangerous Arma 3 Football Manager 2016 Cities: Skylines ARK: Survival Evolved Dark Souls II: Scholar of the First Sin Killing Floor 2 H1Z1 Sid Meier's Civilization V Dying Light Middle-Earth: Shadow of Mordor

Elsewhere in Galyonkin's blog post, data suggests that the rate of new releases on Steam has begun to plateau.

Rate of new Steam releases between Jan 2012 and December 2015.

Valve, which takes roughly a one-third cut from most games sold on Steam, is believed to have made close to $1 billion from all game sales in 2015, if the Steam Spy data is accurate.

Steam is the world's biggest core games platforms. In January, it was revealed that the platform's peak concurrent user number had passed the 12 million threshold.

Galyonkin estimates that more than 350 million games were sold across the platform in 2015.