Some one on r/hearthstone posted this showing that Dragon's Breath does not get reduced from 5 mana with Naga Sea Witch(NSW) on board after killing minions, even though Volcanic Drake does.
Some people brought up Solemn Vigil in the thread. I tested it, and sure enough its cost doesn't get reduced after minions are killed either.
I decided to test some other cost reduction mechanics. Mechwarper, Dragon Consort, Blackwater Pirate, Dread Corsair, Skycapn Kragg, and Happy Ghoul all work, reducing their cost or the cost of cards they effect from 5 mana if NSW is on board.
Kirin Tor Mage & Kabal Lackey both reduced secrets to 0 m


