Any of the classic valve games (half life, portal, and their sequels).
If auto shooters are your thing, vampire survivors is less than a small coffee.
Dead space (the remake) is $15.
Halo MCC is $10.
Titanfall 2 is $3.
The “new” tomb raider trilogy are all less than $5 each or get the bundle.
Alan wake is $5.
Prey is $3.
And Batman Arkham origins is $2.
Just a handful of good games for cheap. There’s many more out there.
Lot of great suggestions here- I grabbed MCC myself. Was kinda stunned that it's... I think five games ranging from absolutely era defining (at least for me) to solid games. For $10 I'm not sure that can be beat.
Second this on Prey. Definitely is a love letter to immersive Sims like System Shock and Deus Ex. Hell there is a rumor that before Bethesda wanted to cover up the homicide of Prey 2, "Prey" 2017 was going to be called Neuroshock. Shame Arkane Austin was forced into live service shit with RedFall that they obviously had zero interest in and most original Arkane alumni left before the inevitable closure when they saw the writing on the wall.