I agree with mps, I just purchased a 6 stage RO/DI 110gpd unit with resevoir and faucet assembly for 130 shipped off ebay. I also purchased a booster pump from them with the selenoid, transformer pressure switch etc and they are gonna send me them together with no additional shipping charge. I think you would be hard pressed to build one for what I've got in it shipped.
110 gpd RO/DI $110.00 buy it now w/resevoir and faucet
110 gpd RO/DI $86.00 buy it now no tank or faucet
85 gpd RO only $96.00 buy it now w/resvoir and faucet
They also have an 85 gpd unit w/out the tank and faucet for a little cheaper, couldn't find it though.