Hi
2020 - I get 115/day as a general short-term supply and 135/day if I were to do long-term assignments (then after 12 weeks I would get whatever official pay grade was).
2000 - I got 85/day when I first started which went up to 115/day as I realised what the going rate was, 135/day for a longer commute, school in unsatisfactory measures or school being OFSTEDed.
The rates have not gone up because of TAs and Cover Supervisors being used everywhere from around 2005.
A cover supervisor salary of around 18K pro rata-ed was the going rate around that time. Their salaries - in fact all admin salaries - have not improved in the last two decades.
That said, even if on better supply rates, because of the lulls you aren't usually on that much more.
Agencies will also lower your rates for admin positions 70/day or for good schools, cover supervisor rates.
Pre-lockdown, in the week before, I could have been booked 5x over. Now there hasn't been that much despite reports of teacher absence - there are graduates who are cheaper, more TA and CS roles around, the Teach First ambassador Covid gap-fill program...all of these will have an impact on number of teacher days available.
Supply was tenable if you were able to be flexible, do many different LEAs and didn't mind the wear and tear on your car or petrol costs. It was also good if you were on more than one agency's lists (but then tax can become tricky).
It provides variety, flexibility and a foot in the door.
It can, however, be hugely stressful with zero job security and you can be left in vulnerable positions financially and practically. It's like doing stand-up and is often a young person's game unless you are recently retired from the school you do supply for.
I think Cover Supervisor is a compromise for those wanting to teach without the planning and prep involved but who want to be seen as a member of staff rather than a commodity.