Thats an extremely generalised question, prices vary from shop to shop. You are best off looking around all the fish shop near you and seeing if they have one. If they don't ask THEM how much it would cost to order one in.
I'm almost horrified that you can get BGKs so cheap. They must be either ruthlessly caught or from very bad quality suppliers......I would actually worry if i wasnt paying under a tenner for a Black Ghost Knife (unless it was like 2", but then i wouldnt want to risk buying it anyway )