$300 for just a DI unit??? That's robbery. Look here...
The Filter Guys
I run a DI unit, no RO. The output is as pure as pure gets and is probably purer than just RO by itself. RO/DI together though will give you pure water also.
I run the water first through a Kold-Steril unit (bad purchase... long story) that takes out heavy metals, big junk, and the chlorine. It then passes through a dual cannister DI unit from the fine gentlemen at the link above. I have a flow meter/valve inline so I can dial in the exact gallons/hour I want flowing through it. Even at flow rates of 60 gallons/hour my 80ppm TDS tap water turns into 0.0ppm TDS pure water. No waste.
Is it for everyone? No. It's expensive. I burn through a DI resin refill about every 5-6 weeks, at around $10-12 a pop. But for me, the ability to have DI water on demand, with no waste, is worth it. Just a personal choice.
Melosu is right though... if you calculate out how many $$ you are spending on that waste water (just assume you throw out 4 gallons for every 1 you make) it's really not that much money and you'd be hard pressed to justify the extra expense of straight DI just on money.
But again... $300 for a DI setup is just plain ridiculous - even if it comes with carbon prefilters, etc.