GOOGLE'S $12.5 billion acquisition of Motorola Mobility seems pricey. The sum amounts to a 63% premium on the ailing device maker's share price before the deal. The purchase is widely regarded as being about Motorola's patents, which Google needs to defend itself from a spate of recent lawsuits.

How might Google, a company famously fond of numbers and maths, have arrived at the amount? The eagle-eyed industry analysts at Frost & Sullivan offer a plausible answer. In a commentary issued on August 16th they note:

"Motorola has a portfolio of 24,500 patents and patent applications that instantly bolsters Google's strength in the IP war. Looking at some recent patent auctions and using some simple math can show why these patents were indeed the target of Google's acquisition. Using one of the industries recent patent auctions as a baseline, in December of 2010, Novell sold off its portfolio of 882 patents for $450 Million. A simple division calculation leads us to a value of $510,204.08 per patent. Why not round that figure off you ask? Well, let's look at the patent value of the Motorola acquisition. Forgetting that Motorola also makes mobile phones, let's say the entire value of the acquisition was in their 24,500 patents and applications. At a $12.5 billion price tag, that equates to…drum roll please…$510,204.08 per patent. Can anyone guess what heuristic they used in the board room in valuing the deal? In the Motorola acquisition, Google bought a patent portfolio and got a mobile phone business thrown in for free."

If Google's acquisition of Motorola was indeed priced solely on a cost-per-patent basis, as looks likely, it would set a benchmark for valuing an intellectual property portfolio. It would also brighten the prospects of other firms that are considering selling their patents. For instance, Kodak, which holds more than 1,000 related to digital imaging, is keen. And it would justify a higher share price for Research in Motion (RIM), the maker of the BlackBerry. The Canadian company has been losing market share for years—but it is sitting on around 2,000 patents, with 3,000 more applications in the pipeline.

Still, valuing intellectual property remains more art than science. In July six IT firms including Microsoft, RIM and Apple paid $4.5 billion for 6,000 patents owned by Nortel, a bankrupt Canadian manufacturer of telecommunications gear. Using the same simple maths, the group paid a clean $750,000 per patent. If Google's latest acquisition was pricey, that one was downright exorbitant.