The reason film camera body is fundamentally dirt cheap is that it requires no hi-tech parts at all. If we rule out lens, lens mounts hardware and mirror box (which could be 1:1 the same for both film and digital), a film camera is essentially at light-tight box that holds film at fixed distance from lens. The part of the construct that captures and stores light information is film, the part you replace regularly. Apart from holding film in place in consistent manner, everything else is just convenience extra.
Digital camera on other hand needs image sensor which is very high tech part containing millions of details that must match at with nanometer level precision, and then signal processor and buffer memory needed to scan, cache and store the image.
Ultimately electronics will keep getting cheaper, because of economics of scale and continued engineering, but still, the default value is that it's very much more expensive to manufacture high tech components than a rail for holding film.
Also, if you look at film SLR cameras, they never costed $6 to make. The reason you get them at that price today is that people prefer digital cameras and want to get rid of old film SLRs.