GOG.com's weekly staff highlights offers five classic FPS games (and one modern FPS done in classic style) for up to 50%-80% off.
The sale covers: Alien Versus Predator Classic 2000, SiN Gold, Duke Nukem 3D Atomic Edition, Unreal Tournament 2004 Editor's Choice Edition, SHOGO: Mobile Armor Division, and Rise of the Triad (2013). No game in that list costs more than $2.99.
If you are looking to brush up on your retro FPS knowledge and experience, this is one of the cheapest (and legal-est) options. The sale will be going on for the next 68 hours. Happy shooting!