Is even the simple answer "as much as someone is willing to pay" correct ? In many cases it may not be - let alone the difficulty of trying to establish what someone is willing to pay !
We find this a fascinating subject and it is a subject in which we have gained extensive knowledge and experience. For example, one of the partners has advised various LLoyds Underwriters on the subject since the early 1980's.
Very many different methods have been devised for the valuation of IP Rights and it will probably not come as a surprise that there are now software products which seek to automate the process. Valuation methods range from the superficial "How much did it cost you to establish the rights" to complex Black Scholes Option Pricing Models (which is fine if you can calculate a number of attributes such as the volatility in pricing of Rights such as the one you are trying to value).
From our knowledge and experience, we can provide you with an overview of the valuation methods available and help you build a properly justifiable valuation of your Intellectual Property Rights.