Minor correction about when Valve started allowing HL2 mods to be sold on Steam it isn’t as clear cut as most people think it is.
There’s two very different paths when it comes to this. The one most people think of is free stand alone mods like Fistful of Frags and No More Room In Hell. This path is very simple, you promise not to sell the game or charge for any part of it through microtransactions etc or else you get in very big trouble. Selling things like soundtracks is fine though.
The other more dangerous path that has already ruined several small studios started by starry eyed modders is the for profit one. Before I get into this here’s the Source Engine distribution FAQ.
Let’s start with the obvious, Valves getting a cut and it is not hard to imagine that they are going to be wanting a hefty one. You are using their engine, their IP, and distributing on their platform. They did most of the work they are going to be wanting most of the money.
Now here comes the part that blind sides most people, middleware. Source is a very modular engine which is really cool because you can just straight up replace the sound system with Wwise or FMOD if you wanted to but it also means it’s relying on a lot of closed source middleware out of the box. Havok wants $25,000 for instance for you using their physics engine. RAD Game Tools is going to want their licensing money as well. There’s also the case as Robert Briscoe of Dear Esther fame found out that a number of licenses are on a per platform basis and that doesn’t just mean PC and the various consoles. That PC category breaks down further into Windows, OSX, and Linux. If you don’t have someone on the team who is an experienced coder who can swap things out for something that is either cheaper or open source you are going to be taking out a very large loan on just the engine.
While having these two points in mind start thinking about things like Prospket that released to negative to middling reviews at a $10 price point. The number of copies you would need to sell to break even is astounding. Unless you can guarantee that you are going to be a Titanfall, Black Mesa, or Insurgency there is no reason you should be using this engine for a game.
So why do people use it? Why are there people who are picking Source over say UE4 or Unity for a project that will most likely land them in debt? Because Valve’s name is attached to it and some feel it will lead to a job at the company like so many other modders before them. The person behind Prospket has talked about numerous times how he was hoping between the very expensive briefcase he mailed them and the game it would get him at least an interview with Valve. So far he hasn’t even gotten a phone call or an email about it.