The price varies from place to place. Personally I will stick with either SpectraPure or Bulk Reef Supply. Plenty of other units out there but these are the two I have most confidence in and can't recall ever seeing any negatives about either. I also like the fact that you see them recommended on many sites for the quality, customer service, etc....
I bought a SpectraPure about two years ago. Everything I could find was positive on the unit and there was plenty of legit data, facts, etc... to support claims and not just some kid saying it was the best unit they had every used (in their 1 month salt water experience). Once Marine Depot started selling the units it seems they have become a little more popular. I know of several locally that use BRS units with no issues.
Be careful buying used RO
units. Most will need filter replacements and end up being more expensive (old unit + new filters) than buying a completely new unit.