Depends what you want.
A decent sh OOF will be around £500 unless vendor is taking the p1ss.
Lots of good Fatties around the grand, mostly compromised in some way – as always build kit is the best scenario to be crap as it’s easily changed. Wheels and tyres are much more expensive than regular mtb, £150 for a pair of tyres is normal if not crap vee rubber or cheap and decent oo floaters.
Pay attention to what is being offered and what you want the bike for. The Fat market is very much split between those wanting trail slack bikes for go anywhere hooning and people wanting steep and tall bikes with gear mount bosses in places you’ve never seen before to attach lots of luggage to and ride in sloppy conditions in more traditional fat bike style (the snow and sand stereotype)
Join a decent fatbike forum and ignore most of what is written about them on STW. 🙂 checkout Slam69 if you haven’t already. I’m running a £500 bitsa OOF right now and deciding if I should spend a lot more replacing it with a much nicer fat bike and probably sell the FS or keep it as a toy. I enjoy all my bikes when I’m riding them, but having cut down to three before the Fatty I’m back in too many bikes not enough time land…