The Fox is a MUCH bigger vehicle, not just the gun.
My wife had the Ferret first and when I asked if I could have a Scorpion the response was, "Sure you can, but I get a Fox too." The Fox is much more pleasant to drive, better EVERYTHING except visibility. There were probably 20-30 Ferrets (4400) made for every Fox (100-200?). The Fox had a production run of less than two years compared almost 20 for the Ferret. The Fox was cancelled in favor of the Scorpion. I would estimate you would have to pay about three times as much for a Fox than a Ferret in similar condition. I see one or two for sale yearly in the US. A few more in the UK but finding one with the original RARDEN gun and proper turret is becoming less common. Most of the Foxes I've seen for sale are pretty poor and they still want $35-40K.