The PhD will only be useful in a small (but growing) subset of data science jobs. Their are data scientists who develop new algorithms and techniques, and those who apply the. For application, the PhD is probably overkill and extensive experience with a bachelor's, or a master's is better. For theory the PhD can't be beat, of course. I say the opportunity costs are high, and should be carefully weighed, because I have my bachelor's and am making more than a data scientist friend of mine who has his PhD. The vast majority of what he learned in the PhD program he hasn't found useful in the real world (yet, anyways). So it's all about what problems you want to work on. Personally I thrive in the application arena of data science, while others thrive in the R&D data science arena. If you like one you probably won't like the other.