I use a razor knife called the
Pyranta Edge by Havalon. It costs about 40 dollars but it works very well for chickens and turkeys. It comes with 13 blades originally and you can get more replacement blades to go with it. I have used it 2 years and still have 6 blades left. I change the blades more to be sure that they will be sharp than because they need it. Usually the blades are still fine and don't really need to be changed but I change it after 50 or so birds just to be on the safe side.
Can't recommend this knife enough for culling and for the actual processing when you have to do it. That plus kill cones makes the process so much easier in my opinion.
Edited because I can't type worth beans this morning.