QIf I play a card for fewer or more play points than it originally costs, and an ability refers to its cost, does it refer to the cost I played it for?

ANo. It refers to the card's original cost—the number indicated on the card itself.
(For example: the "cost" of a Flame Destroyer that was played for 9 less play points is still 9.)

Effects that change how much a card costs "to play" do not change the cost information on the card, only the number of play points required when paying that cost.
Whenever an ability mentions a card's "cost" or how much it "originally costs," it always refers to the information on the actual card. In the case of an evolved follower, it refers to the cost of the unevolved form.