Once it has been replaced with active one, one can bring Ober XP value closer to the current price since their XP value is much higher than the price would indicate.
Their performance justifies that.
Most units exp value is related to unit cost, but there are numerous exceptions from this rule, obers are one of them as their original price of 400mp was split between current mp price and weapon upgrade and we both are very well aware of this. There is no injustice or imbalance here, just a different allocation of resources. End value and therefore exp value is the same.