I have on Omni RO 2000 undersink unit.
http://www.omnifilter.com/undersink.htmI paid around $160 at Menards 8 or so years ago.
How long the filters last depends on your well water. I've only replaced the RO membrane filter once in the 6 or 8years We've had the unit. The guy in the store said to replace it when the water doesn't taste good and the rest of the filters are good. The RO cartridge was about $50. Sometimes the rest of the filters last 6 mos. and sometimes they don't. I brew about once a month 17 gal used per brew. We use the water in our aquarium 10-20 gallons per month. We keep a gallon of water for drinking on the counter for drinking 15-20 gal per month. We use the water for cooking, coffee, tea etc, 10 gal per month.
Let me qualify my filter life by saying my well water has a lot of sulfer and iron in it (smells like rotton eggs). I clorinate my well 3 time a year when. You can do this yourself with a product called Saniwell. The water is filtered through two whole house filters. Once for sediment and once for rust/taste/odor. Then it goes through a water softener. All this makes the tap water pretty decent. I found the RO filters lasted a lot longer when the system was connected to the soft water lineas opposed to one pre softening. I bought a few of those 5 gallon water bottles that the grocery stores sell to bring in and refill because they have a screw on top not like the bottled water company jugs. When the undersink tank is full you get about 1 1/2 gallons out of it. The filtering process is verly slow. I put my water jug in a tub (in case I overfill it) and use some racking hose from the faucet to the jug and leave the faucet on. It probably take 6 to 8 hours to fill the 5 gal jug.
The system has paid for itself as opposed to going to the store for bottled water.