Unless you recharge them, those *lone* DI unit are expensive to buy and replace.

You've seen my setup, I use RO/DI for my reef tank.

http://search.ebay.com/search/search...;query=RO%2FDI

I don't know how this guy on Ebay does it for so cheap.

http://cgi.ebay.com/ws/eBayISAPI.dll...category=20758
United States Shipping and handling US $27.50
Shipping insurance per item (required) US $5.00

His price is even cheaper then AirWaterIce's price. I paid over $200 for my RO/DI setup w/ the RO storage and drinking faucet. He sell out of Canada, so when you convert to US dollar, it might even be cheaper. I'm not sure, probably not.