It probably depends where you are, Popsycal.
I am on the books of an agency based in Berkshire, and their rates are £115 for daily supply (no preparation or marking), and £150 for longer term work.
The international school that I have worked at pays a lot less than this, but the work is much easier (no discipline issues, small class sizes, you only cover for one teacher) so I think it is a good deal.
Surrey schools pays you according to your spine point divided by 190, which is a good deal for experienced teachers doing daily supply.