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I encountered something similar.
It was for Raillag, a blood mage.
As far as I could tell, the cost of spells from the Prime magic school increases with the hero's level – the Haste spell (which is also a specialization for Raillag) costs around 90, and Teleport costs around 500, and he only has about 460 mana.
So, I can't afford Teleport to use it.
Added after 1 minute
If Teleport and Haste belong to the School of Darkness (I don't remember exactly), then it seems that the native school of the faction affects the price.