is a decent rail system that will fit the 870 mag extension. It'll also fit forward of the foreend on a Mossberg 500 5 shot mag tube.
Combine it with a decent 1" scope ring, or a CAA light mount
and a Surefire G2 LED
and you'll come in not much over 100 bucks total.
You could trim some costs by trolling for used versions of the above or by switching to a less expensive flashlight (Streamlight Polytac perhaps?). I have experience with the above though so that's where the recommendation is coming from.