Join a group but here for Bulk Reef Supply. You can save over 20% and you will bet a good unit. I got a $250 6 stage unit for $180 but that is the dell package so it has all the connections needed.
*deluxe package. Err auto correct
trust me, get the BRS one that Phranque listed, it's worth it. That's what I have, has the pressure gauge, membrane flush kit and inline tds meter. and joining the group buy is even better. Mine has been chugging along since I got it
If you cant afford that much try ebay. I saw a portable 100gpd for $66 earlier today. It may not be as good as the ones from BRS but mine worked just fine for nearly a year before I had to change filters ($57). I have over 400 gallons in aquariums (ranging from 1.5 to 90 gallons) and do regular water changes on them. so I cant complain. I did add a BRS dual TDS meter to it when I bought it. That would hold you until you could get a better system.
Why do I love fish? Because every time I stare into the little world I have created for them, every problem I have just floats away....