I'm obviously not saying that any work (of value) on the instructor part should be at no cost for the student but I find that charging as much as dual rate on solo navigations is nothing short of fraud. I'd be happy to be educated to the contrary, that is.
Okijara, I run a one man band flying school and need to earn as much as possible to keep it viable. If I turn up for work and spend the day there I need to get a days wages regardless of whether I sit in the aircraft or not. Often I turn up and due to weather or whatever, I actually earn nothing at all for that day. I commit no fraud whatsoever! It is never what I earn in a day, a week or even a month. What makes my business viable is what I earn in a year. Some days it is north of 1K and the average student would think I was making a fortune. That 1K quickly divides when I turn up for a single hour of dual the following day and the day after is not VFR.
Some of my students are doctors, some are tradesman. both these professions charge more per day and earn more per year. I would argue that I deserve at least as much as a tradesman. I was one once. Sadly I fall way short over 12 months but the lifestyle makes up for it.