Sorry for the delay in pricing up. Having internet issues.
So will work with a 4x4 space as an example.
8x citizen 1212 cob chips in 3000k or 3500k Price about $10 per chip (possibly cheaper from China)
$80
8x 120mm @ $9.50 each
$76
2x Pairui driver LGSU250-C1400D , each will be running 4 cobs. @$37 each
$74
Total price for the big stuff.
$230 ,
Remember that's $ not pounds but depending on where you buy you need to factor in taxes because not of these are available in the UK, although you can buy the cobs from primelight at a very good price which is in UK. The rest the best option for UK folks is Kingbrite on Alibaba. They are tried and tested and reliable and will fluff the value so you don't get your bum touched to hard by the tax man. Normally tax is 20%, ask them to reduce the value on invoice so you pay considerably less.
You also need aluminium angle to build the frame.
http://www.aluminiumwarehouse.co.uk/Aluminium-Angle/c120_124/p2857/3/8__in_x__3/8__in_x__1/16__in_:_Aluminum_Angle/product_info.html
A 5m length should do it all,
£13.62
Wire, if you buy from kingbrite you can ask them to throw it in as part of the deal. They should do it for free. If not its a few quid from UK.
Solder.
Thermal adhesive, again if you buy from kingbrite they will sort you out some thermal pads to stick the Cobs to the Heatsinks.
Wire connectors. A few quid.
Couple of plugs.. Cheap as Chips.
This is a budget build, I havnt included reflectors or lenses because you don't need them. But don't cost much if you wanted to use them.
I priced the Cobs up using gen5 cobs, gen 6 are now available and are even better. The above would be 45% efficient using gen 5 and even better using gen6. All prices are at least 6 month old and I would imaging all the main stuff will be cheaper by now.
The example I have used will light up a 4ft x 4ft area about the same as a 600w HPS but will use 429w at the wall.
There may be cheaper options, I put that together using prices I had at hand. If there are cheaper options it wont be by much.
If you have any questions I will try and answer, I'm not an expert though, I'm a bit of a dunce compared to most of the Cob guys.
@lee6