Cheap effective proximity sensors for detecting people?
IR

(1) LEDs used to wash area from above with modulated IR with cheapest IR detectors that work for you. (LED,photodiode,...)

(2) IF you are able to provide emitters at floor level facing up you can use beam interruption with sensors above. IR emitters can easily be very inobvious (IR filter can be black and opaque to visible light.)  Emitters should be able to be made "walk on" damage resistant.

(3) Emitters above with floor level reflectors - need not be visibly reflective. More liable to be subject to damage than low level emitters.

(4) Alibaba India has active IR sensor boards at Rs157 =~~~ $US3. This is whole PCB and volume unknown. Gives an idea of bottom order of cost for completed units. 



(5) Capacitive may be able to be made to work at your range. 

Philips PCF8883 sounds promising, priced at Digikey at $1.08/2500 or $2.80/1.

If you can install the sensor plates at floor level rather than above the targets then capacitive sensing should be very suitable.

Many capacitive sensor circuits of variable merit here via  Google images



Some well priced PIR sensor components



Ultrasonic sensor pairs
From $US2.90/pair 1's, $US2.30/100's.