Yet another good post from Mark Guzdial pointed me at an article in The Economist about the value (or otherwise) of a PhD. Key stat (bold emphasis mine):
A study in the Journal of Higher Education Policy and Management...shows that British men with a bachelor's degree earn 14% more than those who could have gone to university but chose not to. The earnings premium for a PhD is 26%. But the premium for a master's degree...is almost as high, at 23%. In some subjects the premium for a PhD vanishes entirely. PhDs in maths and computing, social sciences and languages earn no more than those with master's degrees. The premium for a PhD is actually smaller than for a master's degree in engineering and technology, architecture and education. Only in medicine, other sciences, and business and financial studies is it high enough to be worthwhile. Over all subjects, a PhD commands only a 3% premium over a master's degree.