The number was mentioned by an oil exec last week. It pertains to Prudhoe Bay, which is admittedly not cheap oil.
Yeah but he gets paid to say that. Actually there was a great article on Bloomberg about this a few days ago, I'll try to find it. No way that crude costs 50 dollars a barrel, it would never have been feasible to produce it until very recently.
Here is the article.
http://www.bloomberg.com/apps/news?pid=20601039&sid=acUzduABKCDo&refer=columnist_pauly