The most-played game on Steam is free, but that doesn’t mean it isn’t making Valve a ton of money.

Dota 2 is a growing force in the multiplayer online arena battler space. It makes $18 million every month, according to industry-intelligence firm SuperData Research. And that number is on the rise as Valve’s highly competitive game attracts new players and helps the company capture a bigger piece of the MOBA market, which is about $54 million a month in the United States alone.

“The success of the MOBA genre continues as one of Valve’s top-tier titles broke the symbolic threshold of 1 million concurrent users in February,” SuperData Research chief executive Joost van Dreunen wrote in a blog post. “Worldwide, the game has a monthly active user base of well over 10 million players.”

But while Dota 2 is on the rise, it still has a very long way to go to catch up to the dominant MOBA, League of Legends.

“It takes League of Legends about five days to earn that same amount of money [as Dota 2 makes in a month,” said van Dreunen. “Even though earnings have started to plateau around the holiday season, due to the major releases on other platforms, League of Legends still had a monthly run rate of over $123 million in January.”

League of Legends, from developer Riot Games, pulls in that kind of cash thanks to its massive player base. In January 2014, the company said it had around 67 million monthly active players, and it’s likely that figure has ballooned since then. To put things in perspective, League has around 27 million people who play at least one game every day. That’s nearly three times the amount of people who play one Dota 2 match a month.

But despite that disparity, both League of Legends and Dota 2 are doing very well.

“The popularity of the genre has even convinced several legacy publishers to develop their own MOBA titles,” said van Dreunen. “Like the upcoming Heroes of the Storm by Blizzard and the recently announced Supernova by Bandai Namco.”

While Blizzard and Bandai Namco are still chasing after MOBA money, others have actually already given up. Most notably, publisher Electronic Arts canceled its MOBA Dawngate after testing it in open beta for six months. While gamers are spending a ton on the top MOBAs, it’s unlikely that the market can support five or six games beyond the top-tier earners.