Does Rhystic Study trigger if an opponent pays the one mana to avoid it?

Rhystic Study reads: "Whenever an opponent casts a spell, you may draw a card unless that player pays {1}." The payment of {1} is a condition that must be evaluated as the triggered ability would trigger — if the opponent pays, the trigger never occurs in the first place.

Under CR 603.1, a triggered ability triggers when its trigger condition is met. Rhystic Study's trigger condition is effectively "an opponent casts a spell AND does not pay {1}." Because the opponent has the option to pay at the moment the spell is cast, if they pay, the condition is never satisfied and no ability is placed on the stack.

This is different from abilities that trigger and then allow payment to counter or modify them. With Rhystic Study, the choice and payment happen simultaneously with the casting of the spell, so there is no trigger to respond to if the mana is paid (CR 603.2).

Concrete example: Your opponent casts Lightning Bolt and immediately pays {1}. Rhystic Study does not trigger, nothing is placed on the stack, and you do not draw a card. If they had cast the spell without paying, the triggered ability would go on the stack and you would draw a card.

Note that if the opponent says nothing and doesn't pay, you draw. It is good practice in games to ask "pay or not?" clearly to avoid disputes, as Rhystic Study is a "may pay to prevent" trigger rather than a "may pay after trigger" ability.