With respect, the question you should be asking is "how much does it cost to defend a patent?"

I recently attended a presentation entitled "so you think you want a patent" by a leading IP Barrister - it was real eye opener!! and has pretty much put me off ever seeking patent protection!! unless you've got a £150,000+ to back up any potential infringement or IP insurance you are probably wasting your time and money. In my view a patent - for most people - is a ticket for guaranteed future distress and expense. Its the lawyers that rub their hands with glee whenever a patent is granted. There is a very high risk that if you took an infringer to court, the infringer would claim that your patent should never have been granted in the first place - and there is an equally likely chance that the courts would agree and your patent would be taken away - just like that!

There is a well respected Barrister - Jane Lambert who presently has a book at the publishers which you should buy when available!! it'll open you eyes too!!