Does Blind Obedience's extort trigger if you cast a spell for free with no mana cost?

Extort reads: "Whenever you cast a spell, you may pay {W/B}. If you do, each opponent loses 1 life and you gain that much life." The trigger condition is simply casting a spell — it places no restriction on whether mana was paid or what the spell's mana cost is (CR 702.101a).

Casting a spell 'for free' (e.g., via Cascade, Suspend, or a card like Force of Will's alternate cost) still means you are casting a spell. The act of casting is what matters, not what you paid. CR 601.2 defines casting a spell as moving it from wherever it is to the stack, which happens regardless of cost payment.

Once the spell is on the stack and has been fully cast, Extort's triggered ability goes on the stack. At that point, you may choose to pay {W/B}. This payment is optional — you can always decline — but the trigger itself will always occur (CR 702.101b).

Concrete example: You have Blind Obedience on the battlefield and you cast Ancestral Vision off Suspend (paying no mana). Extort still triggers. You may pay {W/B}; if you do, each opponent loses 1 life and you gain 1 life per opponent.

Note: Blind Obedience's Extort reminder text shows {W/B}, meaning only white or black mana can be used to pay the extort cost, but the trigger itself fires freely for any spell cast (CR 107.4c).