Most of the R/O filters being built and sold on Ebay and those specifically for the aquaria industry are made from standard parts. However, there are different grades so you have to compare apples to apples. That being said, you can get a great deal on Ebay. I just purchased a new unit to replace a 13 year old unit. I got it on Ebay for $199 + shipping, and it came with:
75gpd Filmtec OEM membrane (stage 4)
John Guest fittings (generic?)
Clear canisters stages 1-3
1 micron sediment (stage 1)
1 micron carbon blocks (stages 2 & 3)
pressure guage
membrane flush kit
inline TDS meter w/LCD
Drinking water kit with 4 gallon metal tank, faucet and 3/8 lines to faucet and tank.
filter wrench, saddle valve, etc.
It was a very well put together unit, and the quick connect fittings make it a dream to work with.
With the TDS meter, I see that my ingoing water measures around 450ppm and the outgoing water measures around 4-10ppm.
Deionization was an extra $30 option, but I didn't need it.
