www.bulkreefsupply.com has 300 GPD Spartan RO/DI systems for $420. Pretty good price imo. I'm waiting to hear back availability from my LFS since I prefer to buy locally, but if all else fails I will buy the Spartan.
One thing to keep in mind is that it is much better for your filters (particularly the RO and the DI) if you use the system less often for a longer period of time. I have two 65-gallon drums. When one is empty I start using the second one, and refill the first one. There is a float valve to turn it off when it fills up (although usually I am watching and will turn it off before it hits the float valve, but I will not have the system turn itself on...that is recipe for disaster that I've seen too many people go through. In this way, I never make less than about 60 gallons at a time. I use about 80 gallons per week though, and my patience wears thin so I'm looking at upgrading from my current 150 GPD system to 300 GPD so I can fill my new 100-gallon containers faster.
My current system is an AquaFX Barracuda that I replaced the RO with a Filmtec 150 GPD membrane and am using a 75 GPD flow restrictor so I am getting about 1:1.5 good:waste. My RO membrane is about 2.5 years old now and is starting to clog up from not having the recommended amount of waste water flowing over it. It is still only putting out 8 ppm (tap water is 240-300 ppm), but is only running about 120 GPD at 75F and 62 psi. I'm going to try cleaning it with citric acid. That's what I chose to do though since I hate wasting so much water. BRS only charges $65 for the 150 GPD Filmtec membranes so I don't mind replacing it every 2-3 years. I think all the Filmtec membranes suggest 1:4 good:waste which is what you will get if you match the flow restrictor to the membrane GPD.