£170 is an average price.
but i never price without seeing a job and i've done them cheaper if easy to get to etc.
you sure it needs a complete valve and not just an actuator or motor.
many fail to diagnose them properly and just change the whole valve when maybe only a £15 motor is needed.
what make/model is it and why does it need changing. ?
hi-spec plumbing & heating (essex)
February 2010