As a gliding instructor I find the thread really interesting as, in the UK at least, we don't charge extra for supervision/instruction/monitoring. The "cost" of paying for an instructor is built into the overall financial model of the gliding club. Admittedly this works because most gliding operations are clubs and so are often run by volunteers rather than paid staff. At my club we have a mix of paid and volunteer instructors. This works well as it keeps the costs down (and so the charges that the student pays) whilst providing us with sufficient instructors. Don't think this model would work with a commercial flying school though!