I charge £45.00 but as with everything the price varies wherever you go. The highest I've seen a large company charge is £105.00(!) but of course you might find a guy who is desperate for work who will do it much cheaper.
I am a gas safe installer, so if you want me to give you a quote then send me a message with your details and I'll see what I can do. (Even if you just want advice - that's free to Albion supporters of course!)