How can you build or buy a wireless motion sensor for the least money?

Possibly go to Home Depot or Lowes. Radio Shack is a possibility but I think they are expensive anymore and are a joke anymore also. Not sure what the prices would be at Home Depot or Lowes. Northern Tool may be another possibility but I dont know where all Northern Tool has stores, but you could check them out online.

What is the purpose of the sensor?

You might try Sears. Just tell them you need a new one for your garage door opener.

@the100thmonkey to sense if there are any intruders in a given area of my property remotely, and cheaply.

@Ltryptophan , most of those devices operate on infrared. You can buy them at any Ace Hardware store. They’re used to control floodlights. People who have them on their porch and coach lights sort of annoy me, because they switch on when I’m walking my dogs past their house. If I’m on a public sidewalk, there is no reason for those lights to switch on; the street light is adequate.