Hi CyberVIP,
I'm not sure, yet I don't think it is a bug. If you look at the Trarii and see how much armour and arms they have compared to Principles and Hastaii you will see that it is immensely more henceforth causing the upgrade to cost more. Also they cost much more to train in the first place and the game always calculates based on that. Hope this helps, cheers!
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