Firstly, patent offices charge official fees upon submission of an application, for searching for prior art and for examining the application, and so on. They also charge yearly renewal fees to keep the process alive- if the fee is not paid, the patent rights are usually lost irrevocably. This is a major cost. Negotiation on patentability with the patent examiner requires further professional advice and cost from the agent. Nevertheless, professional legal advice is indispensable because often the validity of a patent will hang on the legal and technical interpretation of a single word or phrase.
Patent agents know how to choose the right wording.
The cost of pursuing the application after the original filing is from 2 to 6 thousand pounds depending on how many other countries one wishes to file in. Click here .