a mate of mine has asked if i could start selling pure water to him, this i can do however no idea on what to charge him. any ideas please?
I suppose about £15 per 500 litres isn't that unusual. If it's a mate though, you may only want to recover the costs.
You haven't made your own costs very clear though. RO/DI or just DI? Metered or unmetered supply?
Ignoring initial outlays for kit and just going on consumables for RO/DI based on my own expenses.
Perhaps £80 a year for membranes (avg.).
Similar for pre filters.
Maybe 2 bags of resin a year - £160.
Metered water - £500 a year.
So, the most common consumables at £820 a year - say £16 a week. His water will push up water and resin - and possibly the membrane average. If he has two fill ups a weeks @ £15 a pop you should cover the costs of your own water and consumables.
Your figures will be different to mine. As you have made the initial outlay, getting free consumables from what he pays you and not charging him for your time in getting them, £15 a pop for a 500 litre fill should be fairish.