Valuing patents is not a straightforward exercise, as this article from the Economist explains. Compared to physical assets, the value of patents is multi-dimensional and the appropriate measure will depend upon how it is being used. The subject of the article, PatentVector, has developed an interesting approach to exploring one of these dimensions.
When a patent application is filed for an invention, the patent office will perform a search for "prior art" (existing documents that describe the invention). Most of the documents found in these searches are other patent documents. When your patent is cited as prior art against other patents, other people may be trying to protect your invention. PatentVector is based on the theory that the more often a patent is cited, the more interest there is in protecting the invention and so the more valuable the patent should be.
We can argue with the premise (and I know people will!) but I think this is an interesting approach to exploring the value of patents. It's certainly not the whole picture, but does throw up some interesting insights. According to the article, it is the patents of technology companies that hold the largest aggregate value, but the single most valuable patent (according to PatentVector's analysis) is worth a staggering $44.1 billion, for a surgical saw owned by Johnson & Johnson.