Cabby's can be anything from £500 to £5000+ depending on what state it's in. I'd say between 2-3k for a decent one, 3-4k for a top one and anything above that for one that's had a lot of mods done and maybe an engine transplant.
Cabs are wicked, you'd love it.
I had one for four years and I'd still have it now if I could have afforded to keep it along side my Rallye and my camper.