I think its important to remember that what yo uneed is relatively basic.
1, Battery.
If we are talking about a van mount, then a decent quality leisure battery can be got for between £30 and £80, the more you pay then hopefully the better the quality of the Battery, but a good one is still not going to be that expensive.
2, R/O + filters
You can go for a top of the range 40.40 jobbie and spend the best part of a grand or more on the R/O, the filters and large DI cannister, or yo ucan go for one of the small ones from R/O man and so on.
My own is one such, it has 2 small membranes, a 1 litre DI vessal, ditto with the particulate filter and activated charcoal filter.
It reduces my tap TDS (aproxx 150-180 ppm) down to .001 ppm prior ro going through my DI vessal.
It will fill up my 1000 litre holding tank from empty in a couple of days.
In almost 3 years I've never ran out of water.
The filters are cheap to replace and I've a bag of resin that never seems to go down, been using the same bag for well over 12 months.
Pump.
Er, what can you, they are cheap anyway, and last a long time.
Poles.
Mine are the cheapest on the market I believe (and one of the lightest) The Unger poles, can take me up over 40ft (wouldn't recommend them though if you do a lot of work at that height)
Poles are are old as the rest of the system and still going strong.
Brush.
Up until a month ago I was using the same brush day in and day out, the 15inch Salmon...just the one brush is all I had, and that brush is still fine.
But again, brushes aren't expensive and they last years...er...except the Ionics ones I'm told
Hose.
Well microbore or standard, even for 100m of either it isn't that expensive...even the top quality stuff.
Hosereel.
On this point I'll concede that expensive is way superior.
I started with the hoselock reels, cheap as chips.
But compare that to the big expensive ones and there is no comparison, jsut about the best upgrade I did was going for the Cleantech version of the all steel reel...but even then, it isn't that expensive.
Van tank.
Well the specialist tanks for carrying liquid are not that cheap, unless you go for an 100 litre IBC that is, but you have to have a very comprehensive frame for those as the walls of the IBC tanks are paper thin by comparison to the other kind.
I didn't put together a DIY system, mine was from Peter Fogwill, started out with a basic trolley system and upgraded to a van mount.
Where possible I've used Peter as my supplier, he supplied my tank for the van (and his price was little different than from getting it direct from a specialist tank supplier)
And Peter also supplied the frame too.
I was originally up and running for about £1,500 and I've spent about a grand or so since, upgrading to a van mount, Fibreglassing the van floor, getting a 1000 litre holding tank, submersible transfer pump and buying 100m of tricoflex hose and a large steel hosereel and so on.
But the production end isn't that complex and doesn't have to be very expensive.
If you have the funds then go for the big boys stuff,...it sure looks purdy
At some point I'm going to outgrow my small R/O and will need to upgrade to something that will produce the water quicker, but in 3 months I'll have had it three years, and I've not outgrown it yet.
Ian