I used to charge �16.95 Chimney lashed but that was way back in the eighties, nowadays it'll be more like �50 I should think. But make sure you get a reputable company in to do it or you could get a load of rubbish.
anyhow, I'd say expect to pay about �85 - �150 now depending on ease of access to fit it (it may be more if it's a real problem to fit, get a quote first)