Depends what kind of RO system you have, some have ratios of over 5/1 which means 1 litre of pure water will give off 5 litres of waste water which is going to cost you alot on a water metre.
Mine gives me .6litres of pure water to 0.25 litres of waste water every minute, so i have a very low ratio. Then i am not on a water metre.
Your local water company can supply you with costs, then you can work it out.
Justin