cheap but good?

what is the cheapest but very sensitive alarm system there is

   Whats up? Lets think about what you are asking. When you say "very sensitive" what part or entry point are you tring to protect? If you are talking about general impacts or someone bumping the car it will never work out for you. You will get so tired of the alarm going off you will quit using it. Almost anything loud like thunder, airplanes, loud cars, other peoples stereo systems. The alarm should only trigger if someone brakes in anything else should be a pre-warn.