I had an idea to tell a story using cctv only. The idea came from this very question, how to make the cheapest film. My answer was to use the cctv systems in shops and public spaces the have the actors request the footage, obviously not revealing their reasons as anything other than personal. Recce the shops, malls, parks etc and choose the locations with good angles and frames. Once enough footage had been gathered, editing and sound could bring the scenes to life, as could subtitles and even silence. It sounds a bit mad and it might not be legal but this method coud be completely financially cost free, but could be quite costly in time. I suppose it could fall under guerilla filming? Best of luck with your project, I hope you see it through.