Right now on Steam:
Deus Ex for $5
Deus Ex, Deus Ex 2, and three Hitman games for $17
and some variations thereof (including a bundle of two $5 Deus Ex games for $12, which is a bit of an oops).
Deus Ex is an old FPS which is known for having a good story, some RPG elements, and a significant amount of choice in gameplay (stealth vs. brute force vs. technology). The graphics are blocky by today's standards but I thought it was a good game when I played it last year. I haven't played the sequel, but it's supposedly more of a run of the mill shooter. I haven't played any Hitman games.