Tap tds is 375, come out of the ro at 8 with a 3:1 waste pure ratio, with nearer a 1:1 ratio, slightly more waste than pure the output is nearer 15
Looking at the maths in a back-of-an-envelope manner....
Every 1,000 litres you produce at 3:1 will use 2,000 litres more than at 1:1. 2,000 litres cost including sewerage here is £2.90, so £5.80 more per 1,000 litres.
Using the Gardiners resin calculator, that 1,000 litres of RO output running through your resin with an "excess" TDS of 7 (at £64 a sack) will cost you 82p. Thus, for every 1,000 litres pure at 3:1, you're looking at paying a net £4.98 more.
You also need to factor in the membrane life, but I'm testing that out at the mo. My RO output has gone from 10 to 12/13TDS over two months of producing 1,000 litres a working day (input ~280). That's roughly 40 working days, so I've already saved about £200 in water charges; much cheaper to have a slightly shorter life from the membrane. That doesn't take into account a slowly rising cost of resin in the future; I need to do some more work to calculate the break even point on that one, membrane vs resin.
Lots of holes available for picking in that maths, but should be in the right area.
Hope that helps.
Vin