You could estimate this by comparing it to the dev teams behind all Windows OSes. I don't know the share of MSFT's many tens of thousands of employees that contribute to the OS. Let's say it's just 2000 FTE. I'm told a good startup estimate for FTEs is $15K/mo. 2K * 15K * 12 = $360M. Windows XP had 40M sloc, making for $9 / line.

That makes this at least within the right order of magnitude. Then again, this is an application, not an OS. Also, the linux kernel isn't everything, and XP isn't the only OS to consider, i.e. maybe Windows Mobile should be included too.