"How much will Black Mesa cost?


Black Mesa is a free modification; the developers will never charge you for playing mainly because they wouldn't want to make people pay to play, but also because it would be illegal to do so. As long as you have Steam installed on your computer, Black Mesa will cost you nothing."

wiki.blackmesasource.com/General_FAQ#How_much_will_Black_Mesa_cost.3F

They started to make the mod with goal to make a free remake of Half Life 1
If they wanted to sell it as a game, they will need to pay Valve for the Half Life franchise licence (I doubt Valve will sell it in the first place) and they will have to buy additional licence for Valve Technology aka Source Engine

developer.valvesoftware.com/wiki/Licensing_Valve_Technology







inb4 I get raped by red thumbs