One of the most asked questions a patent attorney hears is "How much is my patent worth?". The easy answer is "Whatever you can get for it". However, in reality, there really is no easy answer. The value can be approximated using sales forcasts and the like, but many variables can make or break such an approximation. These variables can include costs of production, market swings, etc.
To assist (at least a little) in gaining some confidence and credibility in assigning a value, a number of links are provided below to articles, book titles, and other rersources. Although no single resource will help assign a value to a patent, the real "value" of the resources will be promoting a better understanding.
Valuing Innovation, Invention, and Patents
Determine a Patent's Estimated Value
Appraising Inventions: The Key to Technology Management
PatentCafe's IP, Invention and Business Books
PatentValuePredictor.com