Why is it so expensive to produce cameras for non visible light?
It comes down to market size. Where is the demand for such cameras and do the number of sales justify the production set up costs? You can get an infra red conversion to standard type DSLR cameras (eg Do It Yourself Digital Infrared Camera Modification Tutorials) and you can convert the camera to a 'full spectrum' type which takes in some ultra violet. (see Full-spectrum photography). For smaller wavelengths you'll need different sensors. These, by their specialist nature and low volume production, tend to be very expensive.