The cost to redevelop the Linux kernel version 2.6.0 in a traditional 
proprietary development setting has been estimated to be $612 million 
USD (€467 million euro) in 2004 prices using the COCOMO man-month 
estimation model.[54] In 2006, a study funded by the European Union put 
the redevelopment cost of kernel version 2.6.8 higher, at €882 million 
euro ($1.14 billion USD).[55]

This topic was revisited in October 2008 by Amanda McPherson, Brian 
Proffitt and Ron Hale-Evans. Using David A. Wheelers methodology, they 
estimated redevelopment of the 2.6.25 kernel now costs $1.3 billion 
(part of a total $10.8 billion to redevelop Fedora 9). [56]

http://en.wikipedia.org/wiki/Linux_kernel