I bought 2 on Ebay maybe 6-8 yrs ago (4x40). On Ebay I found 2 choices, both SS, ones rated for 250 PSI and ones rated at 300PSI, I bought the 300's. I don't recall the price but they were well under the $1000 price for 2 combined. At this point I'd guess they were maybe 300-350 total for 2. They were new, not used. You can add 2 more 4x40's for more performance.
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.