The cheapest possible mech is likely going to run about $1,000. That's counting 10 DOF (two for each leg, two for aiming gun/camera.) Using Arbotix Commander and a WiFi webcam, a cut-down Defender airsoft gun, laser cut plastics body and stock AX12 servos/brackets. There is no "kit" to do this; at least not yet (I know someone over at Trossen Robotics is working on one!)
Trossen also has a quadroped kit with 3DOF per leg; with all the pieces for moving and control, but no camera/gun/aiming, it runs around $950 IIRC. Add camera/guns/turret to that.