Simply put, I have no mana increase nodes or other causes of increased cost, and it costs 40 mana per second instead of 20 (hovering on the skill shows the 40 point cost. The 20 point cost is only in the description of Insidious Conduction). I equip a -3 mana cost weapon, and it goes down to 37 per second. The only nodes I’ve taken in lightning blast are the two you need to reach insidious conduction (2 in each) and 1 point in the one that changes it to cold. I hope it’s the cost that’s the mistake because this thing sucks mana like crazy!
Just to check, I kept playing until I could get Countenance of Mana, and as stated it reduces the cost by 10, down to 30 per second.