Can someone please answer me this… My explosive trap costs 5 mana… I put on rings that total -7 mana cost reduction and somehow my skill still costs 1.8 mana to cast… Are we using blizzard math now or what? Cause last I knew… 5-7=-2 I could be wrong but I’m fairly certain my math skills are on point with this.
i’d guess you have taken some of the mana efficiency and less mana cost nodes inside the tree and probably nodes that add to to cost, so you’d have to calculate it as a whole, not just the resulting 5 minus the ring mods.
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The flat mana cost reduction gets added to the base cost.
Your base mana cost is 8. You took the Trapas node, so it becomes 11. -7 it becomes 4.
Then you have 2 efficiency nodes for a total of 72% efficiency which only apply to the 4. That would be 2.32 mana, but you possibly have other reduction sources somewhere in your gear. I didn’t want to bother looking at it in detail, just checked your trap skill tree.
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Ahhh okay that makes more sense I was doing my math in the incorrect order, thank you both!!! That definitely helped clear up my confusion about what was going on. 
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In LE, flat modifiers are applied first, then %. “Efficiency” modifiers (& most cooldown modifiers) are old / (1 + efficiency %) = new, such that you need 100% efficiency in order to half the original thing. So if you had a mana cost of 10 & a mana efficiency of 100% your new cost would be 10 / (1 + 100%) = 5.
Also, LE will costs round up for display purposes but use the correct decimal figure for calculations.
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This was fixed in the last patch. Now it shows decimals.
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