a keychain or pen flashlight(LED or bulb) or dollar store or vending machine lazer pointer might be a cheaper simpler option that will last longer. most of the cheap motion detectors work off of the change in light you can test the prop by turning it on in the most completely dark room/closet you can find. with the lights still off does it still work? also if its started in a dark room if you flip the light on in the room after its started does it go off? if it dosen't go off in the dark or goes on when light is introduced it may work to simply point the tightest beam of light you can create at the censor that will be broken when someone walks in front of it like this:

lightsource-------->person----->censor on prop

in some cases it even works just to put a lamp or candle directly opposite so a person approaching casts a shadow onto the censor but it depends on the censors range and how it is housed allot of the props i've seen have the censor buried in a deep whole which makes them less effective as less light gets to them.

if the dark room dosen't work you may be dealing with a Infrared or laser detector in witch case it wont ever help to point a light at it.

a led flashight of the same price would have a much longer shelf life than a throwie LED's arn't like light bulbs and can last forever if used right. like DetroitGrass pointed out you wont have to keep buying new batteries for it and new LED's when they heat up and melt to death.