To charge for refuelling and handling cash is taking things a little too far. No instructor expects to be paid for breathing at the school but a balance that is better than the current state of affairs at many schools is called for and with the much publicised shortage of instructors one hopes this will soon be the case. I am apid a daily retainer and an hourly rate when airbourne. I am quite happy with this arrangement however when I first qualified I was being paid an hourly rate whilst airbourne and no retainer. This was a practically useless arrangement during the winter months because some flyable days were missed as I was not willing to take the gamble of driving the considerable distance to work when the weather was a 50/50 day. That didn't work for the school or me so as I say - a balance is required.
I personally would not wish for the extra hassle that would follow such a clear defined arrangement as the one Athonite describes as it's bound to lead to disagreements and tension. Far better to be more mature about it with an employer who has an ounce of common sense enough to recognise the role is varied and non-standard in general terms therefore a retainer + flight pay is the only way if a salaried position is not possible.
VFE.