Don't forget that TDS is not a bad thing for our tanks.
calcium, magnesium, potassium, sodium, bicarbonates, chlorides and sulfates
Except for the last two, many reefers spend lots of money to add these trace elements to their systems. RO/DI water is just extra insurance against getting too many of any particular dissolved solid in your water. I think the biggest fear is getting heavy metals in your tank. These typically come from iron pipe which is not heavily used in most of Central Texas.
I love my RO/DI and don't plan on giving it up but I have run many a successful tank on treated tap water. Leftover chlorine, chloramines, ammonias, etc that are used in water treatment are a bad thing but can easily be removed with a simple chemical treatment. It is easy to make an arguement for using tap water due to the high calcium levels that we have in the local water.
All that being said....Melev and Buckeye both have good units for good prices.