Supply and demand (via Louis Gray)
Microsoft has purchased 666,624 IPv4 addresses from the bankrupt networking vendor Nortel Netorks after a period of competitive bidding.
The 7.5 million dollars that Microsoft spent on the IPv4 addresses roughly breaks down to a per-unit price of $11.25. In perspective, that is cheaper than the price of registering a domain name, which usually costs around $10 depending on the registrar used.
Why was Microsoft interested in the IPv4 addresses? Because as a commodity they are nearly gone. In fact, the last batch has been issued, meaning that they are now, in effect, a scarce resource. There will be a nearly infinite number of IPv6 addresses available in the future, but complications with the transition to IPv6 has put a premium on the value of extant, and unused, IPv4 addresses.