If you can handle dying a lot while learning the ins and outs of the world, S.T.A.L.K.E.R.: Anomaly is a wonderful package completely for free. Especially if you add the G.A.M.M.A. modpack to it, makes the game play significantly deeper, much harder (in some bullshit and also fair ways), but also just crazy immersive; makes you feel like the actions you take do matter, but if you were to die, you'd just be another loot bag for some other Stalker to come across