Rates vary depending upon which authority you are with, where we are the water supply charges are £30.00 pa plus £0.8911 per cubic metre, sewerage water is charged at £66.60 plus £0.8495 per cubic metre.
Commercial is charged on the size of your inlet pipe.
Without trying to be condescending a cubic metre is 1000 ltrs
If you are running something like a merlin RO then your sewerage rates will be high due to the inefficiencies of that type of RO, generally around 1:6, pure to waste, worth looking at a 40" RO with a 1:1 waste ratio