The ~$300 figure is based on getting from the tap to the tank--with a reasonable level of quality and efficiency.
You can buy a 5-6 stage RODI unit off ebay for probably $100 shipped,
but you still have to get the water to the tank. With discus you are going to need an RO holding tank, a pump to get it out of that tank and into the discus tank. Plus tubing/hose, etc. If your household tap pressure is below ~40psi--then you'll want/need a pressure pump (quality=~$130 shipped). By the time you add up the things that you will probably need/want---~$300 is fair.....
