I've read that most of the cost is in the filters, as opposed to the housing, so I honestly don't think you will save much with DIY.
I would check Aquatic Reef systems for filter prices.
http://www.aquaticreefsystems.com/Aq...ccessories.htm
Here's prices on most of the stuff you would buy.
Just FYI, I purchased my 100gpd 5 stage unit on ebay for $125 shipped. I think it would be hard to build it for cheaper than that.
Mike