Probably would be best to check what your local water company charges for a cube of water, double it and add for delivery, that would seem fair to me.
More than double.
You have to pay for supply of the water you waste as well as the pure you produce, and then you have to pay for sewerage, which is usually more than the cost of supply so it actually costs you (say) supply @ £5 per 1000lt = £20 (at ratio 3:1 4000lt required to produce 1000lt pure) sewerage @ £7 per 1000lt = £21 (you can get a discount if you can prove a percentage doesn't go back down the drain)
So to produce 1000lt of pure will cost £41 (It's actually much less, I just said £5 for illustration purposes)
Then you have to add in the running costs of your system (filters, resin, wear on membrane, electricity for pump etc)
Think it through before you start paying someone to take your pure water away :