The source engine is only free for non-commercial use, as with engines like UDK. This is becoming a standard, with many engines now having "free for non-commercial" models, some limited, some not. In the case of Valve, I would assume they work it much like Epic does with the UDK, there is a fairly affordable license fee (UDK is $100), and you are allowed to make a set amount before any royalties are due (UDK is $50,000) and after that a royalty hits on any other profit (25% to Epic for UDK). Valve requires you to contact them for licensing information, so I don't just know the numbers http://source.valvesoftware.com/licensing.php Selling on Steam is essentially a per item basis, you go through the approval process, and then the terms are based on exactly what is agreed upon for your product. You can check here for the details http://www.steampowered.com/steamworks/index.php In the case of this engine, it is a modified one, and so it is less Raven you would need to worry about (especially if you made your own assets) and the original developers, this being based off the Quake engine I am assuming that would be id Software. This Basically... Honestly though, this is a decade old engine, and it was a sort of hack'n'slash modification of an existing one at that. With all of the other available, and surprisingly affordable options for SDKs and Engines, why would you really want/need to? Especially when you consider you would already need to make and code EVERYTHING else in the game.