Valve's multiplayer FPS Team Fortress 2 has seen its user base increase by a factor of five since it adopted the free-to-play model earlier this year.
Valve boss Gabe Newell made the revelation at a tech event in Seattle, as reported by GeekWire.
He added that the proportion of those users who go on to spend money in the game is leagues ahead of the free-to-play average.
"When we talk to partners who do free-to-play," he explained, "a lot of people see about a two to three per cent conversion rate of the people in their audience who actually buy something, and then with Team Fortress 2, which looks more like Arkham Asylum in terms of the user profile and the content, we see about a 20 to 30 percent conversion rate of people who are playing those games who buy something.
"So that's a fairly surprising and fairly recent statistic, which is that there seems to be something about the content that significantly changes how your monetisation occurs, with apparently much broader participation than you would see out of something like FarmVille."