Least expensive(and really a good way to do it) is install a piece of add on rail on the forearm. The ones that go through the vent holes on A2/CAR
forearms work well for this although you may need a longer screw or nut & washer depending on how they are set up.
A weaver top mount ring and 1" light that can handle the recoil are about as low cost of an option available.
Still assuming Budget - these are surprisingly good lights:
800 Lumens @ 50.00 http://www.lapolicegear.com/lapg-operator-l1-flashlight.html (I have several of the lapg operator lights in differing lumens - no problem - I have too many weapons to spend 200.00 plus a light plus I carry them when dog walking)