If your using the TDS meter from AquaFX that meter reads 0.00. So if you are reading 600 it is probably 6.00, which is 6 PPM. Sounds a little high still if this is the new unit. How many gallons have you run through it? 6ppm is pretty low, but it should be a little lower than that on a new unit, I think.
As for the cheap units, you get what you pay for. AquaFX gives great customer service, fast response, lots of extras on the units ( ball valves, pressure gauges, aluminum bracket, high quality cannisters, and better quality filters). Also check that the membrane passed US standards. If the membrane came in from out of the country they can buy them for much cheaper. The mebranes must pass tests in the U.S., and the ones that don't pass are shipped overseas for sale, then they make there way back from overseas and re-sold at low prices. So you are getting a substandard membrane. You must compare apples to apples. AquaFX makes the Mercedes, and the cheap ones are the Geo Prizm. It all depends on what you are looking for I suppose. Marianne at AquaFX will back here product with no questions asked! It is very hard to find this kind of customer service today! I'm sticking with AquaFX!