Mmm, that would be an interesting question to debate at length. The Econ PhD will open doors for you that an MPA/MPP won't. If you're looking for some of the heavier duty public policy jobs in think tanks or places like the CBO, OMB, etc., the PhD will do very well by you. Otoh, as you say, the opportunity cost is non-trivial though it may look more trivial at age 50 than it did at age 30.
I suppose it comes down to exactly where & what you want to be doing in public policy. Fwiw, Econ History, often looked down upon, is a useful field to have in your repertoire paired with Public if you're interested in Public Policy.