Is this mana cost a bug or intended?

So I've been experimenting with the new unique Indigon, and I decided to try stacking mana and mana regen with hierophant totems. I figured I could self cast a spell extremely quickly to build up stacks, but I have a lot of reduced mana cost nodes on the tree + the hierophant's sanctuary of thought, which reduces mana costs by 50% and another 50% while on full energy shield. To counter these reductions, I got Doedre's malevolence, which adds 50 to the cost of any skill. Weirdly enough my spells are all free, even with Doedre's, as long as I'm at full energy shield.

My limited understanding was that Doedre costs were applied after all the multipliers to the base mana cost. Even if the skill costs 0 mana, with Doedre it should cost 50, right? I know Doedre's modifies even the default attack. Is sanctuary of thought actually multiplied after Doedre?

Also this build is pretty damn awkward so far and would not recommend.

This game is opposite to mathematic rules: additions and subtractions first, multipliers afterwards.
Y=x+50-((x+50)×0.5+(x+50)×0.5)=x+50-(x+50)=0
Where x is your total mana cost before doedre gloves and sanctuary of thought and Y is your final mana cost.

That's not how Elreon's rings work, though. They apply after skill gem multipliers, otherwise they would be ridiculous. Why would Doedre's apply at a different time?

So you can get 0 mana cost skills by doing this, in the mean while? Sounds really tasty! Is it wrong to "abuse" this mechanic until it's fixed? I kind of want to do a build now that utilizes this "bug".

"0 cost skills" is a super interesting mechanic, you guys should consider throwing a unique in that does it at some substantial cost? I'd love to have to not worry about regen / leech / bloodmagic.

Mark is saying that the Doedre thing is a bug, not the 100% mana reduction thing.

100% mana cost reduction, and therefore zero-mana skills, is not a bug. It's a normal thing the Hierophant can do pretty easily. If you reduce your mana costs by 100%, then you're left paying 0% of the mana cost of the skill - which would be zero.

Normal game logic, has worked since forever.

Thanks for the replies and clarification everyone. Glad to know I'm not going crazy.