Depends on if it was just a light hack or more fast hacks/jumping small logs etc. But no more than 3k for the perfect one. To be honest if I wanted a horse for hacking only I would probably look for one on perminant loan that is semi retired or retired from competition as there seem to be a few out there at the moment!
A older horse or only able to hack due to previous injury I'd only want to spend upto £1500. If you look for a more able horse then the price can go up accordingly.
If you only wanted to do light hacking, (ie, regular, but about an hour or less each hack, and mostly at a steady pace) then you could consider horses which although wouldn't pass a 5 stage vetting, would still be fit for purpose.
For example there is a most wonderful little horse for sale at out yard, smart, sweet, bombpoof but has a spavin (which he is currently sound on) and had a sarcoid removed last year. If he did not have the arthriticy hock and never had the sarcoid, he would be 4 or 5 times the price that he is now. He is still fine as a light hack, but would not be suitable for a competitive home.
I mainly hack as my mare has the wrong temperament to be a competition horse even though this is what she was bred for. She is very well bred and has no soundness issues. I can jump her for fun at home and in lessons. I only jump small fences (my choice) and she can school beautifully. I bought her from the stud that bred her as she needed a happy hack home for £1500. She was 7 when I bought her.
I would not consider a horse with soundness issues even as a hack.