CCI/Speer says they're ballistically identical to the 250, FWIW.
Then why does the Mil spec # 34 cost $4 more on average per K?
Did you intend to ask this question of ATK, the present owners of CCI & Speer?
My off-hand guess is that it costs more because they can get more for it. People don't tend to do much research or checking on things, they tend to just look at names, packaging, and maybe some ad copy. So, they see 'mil-spec' or something about suitability in mil-surp rifles, and want it. Even if it costs a little more. That's what marketing is.