Yes it is. I paid probably $350+ for mine. Granted, mine has some additional features, but they're still not cheap. I think a replacement set of pre-filters is between $10-30. A replacement RO membrane is $70-100 depending on capacity (gallons per day). I think I replaced my membrane after a year or so, but I don't think it was necessary. I couldn't even tell if the other filters were dirty, maybe my water is cleaner than I thought.
For those who don't know, an RO membrane looks a little like wide foam adhesive tape rolled around a pipe. The membrane basically allows water molecules through and nothing else. The membrane goes into a plastic chamber with three water connections. One is centered on the end, the other two are on the side, one at each end. Water enters through one of the side connections and floods around the membrane. Pure water transfers through the membrane to the center where it drains out through the center pipe to the hose on the end of the chamber. Water that doesn't go through exits through the other side connection to a drain.
Modern RO membranes are made of a material that is damaged by chlorine so you must pre-filter the water with a carbon filter to remove the chlorine. You also use one or more mechanical filters before the RO unit to remove any debris that could clog up the RO membrane.
In a RO system for the home, the clean water is often pushed into a pressurized tank where it sits until it's needed. It may go from there through an additional filter and/or a UV unit to kill any bugs that made it through the RO membrane or grew in the tank.
Membranes are rated in gallons per day. I bought a 70gpd unit. Mine has a special pump on it that uses the waste water pressure to turn a pump that increases the pressure of the incoming water which serves to make the membrane more efficient. Additionally, because I live on the 8th floor of an apartment building, my water pressure is lower than it might otherwise be and therefore it's a slower process than I'd like, so I was able to add an electronic pump on the inlet side to further increase the pressure, and when the number of tanks I wanted to service increased, I added a second RO housing and membrane so the first one drains into the second and the two function in parallel. I'm able to fill a 55gallon drum at least 2 times per day now.
My RO unit empties into a 55g drum that has a mechanical float valve like the one in your toilet tank. I use a well pump from harbor frieght to pump the water from there into my tanks. It has a pressure tank and when I open the valve the pressure drops and the motor kicks on automatically. It feeds an auto-fill system on my 240g tank and two other filling stations I have plumbed.
If you come to our meeting this month, I can show all of this and more and one of these days maybe I'll draw up some diagrams to better explain it all.
Keep an eye out for the announcement here, but I believe it is Saturday the 20th at my place in downtown Dallas. In the meantime, you can see some of my setup at my blog: http://aquaticobsession.blogspot.com/