The X's in a spells cost are counted as having the value chosen for it while that spell is on the stack. As such, Hangarback Walker for X=1 means a CMC of 1+1=2, and it would trigger Eidolon, while X=2 means CMC 2+2=4 to evade Eidolon's trigger.
Tag: Eidolon of the Great Revel
Eidolon of the Great Revel vs. Delve spells.
Delve doesn't actually reduce the cost of a spell; it just changes how you can pay the cost. The converted mana cost of the spell will remain whatever is printed in the top right corner no matter what!