OK maybe $4. The author shows how to use a dollar store laser and a photo resistor and some resistors/caps with a speaker to make an alarm that goes off when the laser is interrupted. Cool idea and easy to build.
Why put this under weapons? Well I could totally see making a water or paintball bomb with this.
http://www.instructables.com/id/Simple-and-portable-Laser-Tripwire-for-under-3/