Half-Life mod cum standalone title Natural Selection 2 has made over $1 million in its first week after launch.
Over 144,000 copies have sold for the title, developed by San Francisco-based studio Unknown Worlds, with the biggest market being the US.
Belgium had the highest percentage of customers who bought the four-pack, gifting the other copies to friends at a discounted price.
Iceland had the highest sales per capita of population, and Singapore users were most likely to buy the deluxe edition.
Modding has proven an effective way for small teams to break into the market.
Atlanta studio Tripwire won fame with Make Something Unreal champion Red Orchestra, and subsequently released FPS Killing Floor as a mod before taking it to a full standalone version.
Counter-Strike also began as a Half-Life mod before being given a life of its own at Valve.
Most recently DayZ, a hard-core zombie survival mod for Bohemia Interactive's military sim ArmA II, became an internet phenomenon.
The mod boasts nearly 1.7 million players, and put ArmA II Combined Operations at the top of Steam charts for months.
Bohemia is currently working on a standalone version of the game, with development led by DayZ creator Dean Hall.