I have watched F4s sell mint for $600-800+ from camera stores. They did have a warranty so I would say $450-550. I have a F4 that I picked up couple of years ago for $350 and it was mint-. I have noticed that prices of film camera equipment goes up and down alot. This is just an opinion on what I think it is worth. If I wanted a second one I would look for a EX for around $300. Hard to say if fim cameras will go down much more but I think they are starting to bottom out on price.
I just purchased a near-mint 251XXXX from a franchise...Originally they wanted $499 Cdn....Then when I showed I was interested they said "Let me see what I can do"....I took it home for $350Cdn with a 6 month warranty, batteries and strap thrown in.
Got mine for $200 from Glazer's Camera in Seattle. Isn't 100% mint, since it has some slight wear, but one of the repair guys I deal with was a bit jealous, since mine was a lot nicer than his was. Mine is a 249xxxx body with the updated hot shoe...
I recently purchased an MF-23 off of the auction site, brand new in box for $112.00. I forget the vendor, but they have them all the time. The instructions were in Japanese, so I downloaded the manual from MIR.