It's a unit, comprised of several filter chambers.
They are measured by gallons per day. A 100 gallon unit will produce about 5 gallons every hour, hour and a half. A 10 gallon unit will take half a day to produce 5 gallons...
So, depending on how big your tank is, you want a unit that can keep up with the amount of water you need for top-offs and water changes.
The best deal I have found is the 100gpd unit that Marc Levenson sells. Nothing worth having in this hobby is cheap unless you can find something nice that's used. I would plan on spending $200 on a good RO unit.