Here’s some mysterious data. According to Emolument, a website that trades in “crowdsourced pay data,” while people with a BA in philosophy earn, on average, £42,000 (roughly $60,000), those with an MA in philosophy earn, on average £61,000 (roughly $88,000). That’s about a 45% salary increase.
It is not clear whether those with Masters degrees in philosophy reporting their salary to Emolument also hold PhDs in philosophy. Some of the language on the page suggests they do not, but it isn’t stated explicitly, and no separate figure is listed for those with PhDs.
Additionally, as usual, the causal story is unknown. Owing to philosophy being a majority-male discipline, the numbers may simply reflect the fact that men, on average, earn more money than women.
See here for more information.