Thunder breaker mana consumption(?)

[skadad]

On my TB I have 1 point in Ascension and Thunder respectively. When I use them separately they cost as much mana as they say they're supposed to (22 and 27 respectively). If I use Ascension into Thunder; Thunder costs 100 mana.

I've not mained a TB nor really played one on official servers but I cannot see any reason as to why the mana cost is nearly quadruple that of the tooltip.

Thanks,
Best wishes
Skadad