That's crazy money for a rainsoft system in a pre-plumbed house! I paid $3k for a top quality (ie: not rainsoft) system which includes Fleck 5600 valves, 2cu ft of resin, brine tank, separate carbon tank, 2 metal bypasses, and a remote install which required the guy to dig a 200ft trench by hand and lay 1" pipe both ways! I didn't want the system taking up space in my garage, so I had it installed at the back of my poperty behind an out-building.
Before having anyone do anything with a water softner though, you should have your water tested. The water quality is what determines the size and type of system you need. ie: how many grains of hardness, what's the chlorine level, etc ... I'm on Chisolm Trail water and have 18g of hardness out here with quite a bit of chlorine, which is one of the reasons I went with the system I choose.
Here's a bit of research I did when I was looking for a system: http://forum.doityourself.com/archiv.../t-387035.html
I ended up using a guy named Tommy Dorsey for the puchase and install, and had a good experience with him.