Mine £750 inc VAT and initial consultation (I paid £150 + VAT for that but as they took the work on, they counted it towards the naturalisation fee).
They are a good one too, with High Court litigation experience, pure immigration specialists, have the highest expertise rating by the Law Society. They are based in outer London and don't even have a website, they also take on legal aid work.
The fancy ones in Central London with less experience can charge double, easily.