Cheapest I've seen is to take a radiator hose clamp and attach it to the side of the tailgate that allows you to remove the tailgate, it takes them longer to steal your tailgate.
Then I tried the PopNLoc on my last truck, got tired of going to the cab and turning the truck off to get the key to unlock the tailgate.
The hose clamp trick is what I have used on all my trucks tailgate and knock on wood, I have never had one stolen yet. In addition I do have the Pop N Lock. The object is to slow them down enough so that they may just pass on your tailgate and go find another easier one.