It seems to currently be a tooltip bug, because with the hungarlings on you can see the cost of towers go from 400 mana down to 300 mana which is the 25% aura, even though the aura icon you recieve states it's a 35% aura.

Once you cast towers and it's on cooldown (this goes for any spell/skill) it reverts the mana cost back to it's original number due to the cooldown, the most reasonable way to explain this is because when the spell/skill is active and ready it applies the aura, when it's on cooldown or inactive the aura doesn't apply because it doesn't have to.

Either way the aura is working and the aura is 25% of the normal cost.  Hope this helps.