A 5 stage vetting should cost you approx £250 including the call out, although it does vary, so check with the vet first and they will give you an approximate cost. A friend got caught out recently when she was charged £440 (gulp) and the horse didn't pass! Another vet vetted her next potential purchase and charged £225!
It does seem to vary from practice to practice and where you are in the UK. I had a horse 5 vetted in the NE last year and it cost me £180 with bloods. Here in the SW it would have been more like £250.