Suggest a cheap motion sensor

Please suggest a cheap motion sensor for the purpose of detecting occupancy in a room.
The cheapes I have found yet is this: http://robokits.co.in/shop/index.php?main_page=product_info&products_id=234

That sensor will tell you if there is infrared motion in the range of the sensor. It will not tell you if a person is in the room, sitting very still (or sleeping). It will not tell you how many people are in the room.

What is your real requirement?

I only need to detect occupancy in the room. If there is a person or be it person(s) (their number don't matter), I want to switch on the lights and fans, and if no one is in the room for say 10 minutes then switch off?

Does this sensor detect motion or heat signature? I mean I was thinking that if a person is sitting still in the room, the sensor will think that the room is empty, isn't that the case?

Does this sensor detect motion or heat signature?

It detects the motion of the heat.

Thanks. One last question- Motion behind a thick room wall will not produce any output on the sensor?

Correct