Can you cast Swords to Plowshares in response to High Market's sac effect being activated to stop the creature from going to the graveyard? (Been dealing with lots of Academy Rectors lately)
The reason you can't respond to a cost, is because that is what has to happen in order for the ability to go on the stack. So by the time the ability can physically be responded to, the cost is paid the creature is nowhere to be seen. The sacrificing of the creature doesn't use the stack at all, like playing a land or activating a mana ability.
see official rules here.