I just thought I should let you know:
In the past two years I've been working on a FPS Engine. It is now possible to purchase a License for a little price. It's a simple single player game engine with a few unique FX and features for Win32 DX7 and later. You can try a Demo and check EULA, Screenies etc. at: http://www.melog.ch/dropper/
The engine cannot competite with the real big ones, but for the money you'll get a nice product that allows you to release fully comercial stuff without the usual restrictions or royalty fees.