Why is it a rip off?
I'm not saying that £25 per 500 litres is a rip off myself.
However, it's reasonable and logical to base the price below what the buyer could produce it for themselves if using DI only.
Assuming TDS of 200, a bag of resin would produce around 2,500 litres. So, after laying out for two DI vessels, the water would cost him around around £75 (a 25k bag of resin) per 2,500 litres. That's £30 per 1,000 litres. I reckon you would need to give a saving on that to make it reasonable.
Of course, 200 TDS is only an example. If you are in a much higher TDS area the numbers would change a lot.