According to this BBC article here: BBC News - Microsoft spends $7.5m on net addresses
Microsoft have brought 666,624 IPv4 adresses from bankrupt US company Nortel for $7.5 million. About $11.25 per address. More significatly, according to the beeb the final batch of IP addresses which were released earlier this year will be used up by the end of 2011. I think I'd best check if my home router is IPv6 compatible!