FPS Creator Free
Full user review
"I recommend FPS Creator if you want EASY game creation!"
You can't get more affordable that free. No instructions required and you can have your first level up and running in less than a minute. A built-in on-line store allows you to browse and download free game models and music too.
The FPS games are limited to mainly indoor scenes, with only a few exterior elements to choose from. Would be nice to see landscapes, vehicles and destructible scenery at some point.