When I was in High School, I worked in a store that had these black plastic half spheres that would stick out of the celling. It would give people the perception that there were cameras in the store. It seemed to work, we never really had a theft problem (except for the people that worked there

) Maybe that could be a cheap solution for you guys, but the whole key is to have them installed when no one is there. And more important no one can find out that there are really no camera's. If the person thinks they are being watched, the thefts will probably stop.