this is again the retooling problem, as i already pointed out here in some previous post with adequate math. (here:
http://forum.paradoxplaza.com/forum/showthread.php?619602-retooling-divs-with-brigades )
I already pointed out an easy (tough still not very accurate) solution, and the more complicated resolution which would guarantee the price for being the same for the same builds.
al3lka maybe tried to say if he wants to build 1 militia-ac it will be more costly than if you build the same division and the same brigade separately. what is true.
(without any slider, minister, tech etc modifier) (ICD=IndustrialCapacityDays)
1mil 30days, 2.7IC = 81 ICD
1ac 40days retooling plus 60days, 3IC = 300ICD
total cost if builded separately = 381 ICD
1mil-ac 40days retooling plus 30days, 8.7IC = 609 ICD
So you have builded the same thing together for 609 ICD and separately for 381 ICD. I think it is a very serious issue. It encourages (unnecessary) micromanagment what is not a good thing imo.
It is a worst case scenario with militia (no retooling) but you always pay more if you build brigades and divisions together. especially at production start and when "upgrading the line".
All this mess is caused by the retooling time (introduced in AoD).