I have two ready made or if you like factory fitted systems. No1 is a 600 ltr one and No2 (The latest) is a 650 ltr.
There are a lot of variables that will determine the total price. No1 will be the RO system and if you need to make water in the van for a 750+ ltr system you definately need to go for a 40 inch RO system. Dont get caught up in the hype about the waste to pure ratio of 40 inch systems. Some claim 80% pure with only 20% waste. You can run it at that ratio but you will kill your membrane and your TDS out will be high. A 40 inch needs to be run at around 50/50 waste to pure. The other big variable will be the poles you get. Dont even think about going over 24ft with fibreglass ones.
My 2nd .. the 650 cost half of the 600 and I got better poles.
I could say mine cost £3500 but unless you compare exactly like for like in all components it means nothing.
Both my systems have done what they say on the tin and I cant complain about paying twice as much for one as the other. Both companies have given me great service and the systems pay for themselves many times over.
I've just ordered another system from supplier No2.. its a 400 litre one man system. I would think you could get a very decent 750 to 800 2 man system fitted for around the £4k mark to include a decent set of poles to get you to say 3rd storey height. Any higher and you need to get full carbon fibre poles really so put on around £4 to 500 for this per pole for thge high work if you want to do it.
I know this isnt exactly what you would want as an answer but it's the most honest I can give you.
And ordering a factory fitted system the day you get you moolah is going to mean you will wait a while to get the busy companies to fit you in to install your new toy
I hope it helps