If the spell mastery ability applies, you’ll scry 2 even if the controller of the spell pays {3}.

In one unusual situation, you can cast Calculated Dismissal targeting an instant or sorcery spell you control while there is one instant or sorcery card in your graveyard. In this situation, if you decline to pay {3}, the spell will be countered and put into your graveyard. The spell mastery ability will then apply and you’ll scry 2.

Check to see if there are two or more instant and/or sorcery cards in your graveyard as the spell resolves to determine whether the spell mastery ability applies. The spell itself won’t count because it’s still on the stack as you make this check.