Playing as Scourge Queen Zagara at level 15 and 0 mastery.
After researching the Scourge gas cost upgrade, the gas cost completely disappeared; Scourge only costs 23 minerals. Was rolling in tons of gas for the rest of the game.
this is because the -gas tech is actually refunding u 25gas per scourge, but if u make one egg costing 75gas hatched to 3 scourge, well it should be gas free.