It will also depend on how much water you plan to be using...the fillters are not cheap unless you find deals on them online or alternates to the LFS sold items. The membrane filters are pricey but lowes has a version that may be usable for much less money...also at issue is whether your water supplier uses chloramin in as a additive...standard filters do not remove that, you can get ones to remove it though.
I recently purchased a spectrapure unit, not the really fancy one though. It was about $150 on sale. It was rated at 90 GPD, but the thing is about three times as fast as that. Maybe I am doing something wrong, but it is much faster than the rating. This is the one that I got (CSPDI 90).
Wow.... I guess you guys havent been looking at the FS threads or you just want to buy new. I just traded my old RO/DI unit to Robert... There is also one that Hobbz is trying to sell. I was asking 60$ for mine and I think Hobbz is looking for 40$. Biggest thing is how much GPD your looking for. I never had a need for anything more than 50-75gpd. My new one I bought from a guy in Orlando is a 7 stage with dual DI cannisters with all the bells and whistles (in line tds, backwash valve, stainless fittings. Honestly my new one produces .000 tds... the old 4 stage did too lol! SO I am not sold that more filtration means better and in some cases I think you loose performance with the additional cannisters. Get yourself a TDS meter and measure your water source you plan on running the RO/DI filter on. The higher your initial TDS the more filtration you will need.