"Free" With Taxes
Judge Level 1Force of Will for 0 mana? Not while Thalia is watching.
Cards involved
The trap
Your opponent controls Thalia, Guardian of Thraben ("Noncreature spells cost 
more to cast"). They cast something threatening — you are fully tapped out but hold Force of Will and a blue card.
What happens?
Does Force of Will save you?
You want to pay the alternative cost: exile a blue card and pay 1 life. What does the spell really cost?
Pick an answer to continue.
The total cost is built in a fixed order: first the alternative cost replaces the mana cost, THEN additional costs, increases and reductions are applied (601.2f). Thalia's tax hits "free" spells too.
601.2fThe player determines the total cost of the spell. Usually this is just the mana cost. Some spells have additional or alternative costs. Some effects may increase or reduce the cost to pay, or may provide other alternative costs. Costs may include paying mana, tapping permanents, sacrificing permanents, discarding cards, and so on. The total cost is the mana cost or alternative cost (as determined in rule 601.2b), plus all additional costs and cost increases, and minus all cost reductions. If multiple cost reductions apply, the player may apply them in any order. If the mana component of the total cost is reduced to nothing by cost reduction effects, it is considered to be 
. It can’t be reduced to less than . Once the total cost is determined, any effects that directly affect the total cost are applied. Then the resulting total cost becomes “locked in.” If effects would change the total cost after this time, they have no effect.
The math
Exile a blue card + 1 life + — and you don't have the . The threatening spell resolves undisturbed.
601.2fThe player determines the total cost of the spell. Usually this is just the mana cost. Some spells have additional or alternative costs. Some effects may increase or reduce the cost to pay, or may provide other alternative costs. Costs may include paying mana, tapping permanents, sacrificing permanents, discarding cards, and so on. The total cost is the mana cost or alternative cost (as determined in rule 601.2b), plus all additional costs and cost increases, and minus all cost reductions. If multiple cost reductions apply, the player may apply them in any order. If the mana component of the total cost is reduced to nothing by cost reduction effects, it is considered to be . It can’t be reduced to less than . Once the total cost is determined, any effects that directly affect the total cost are applied. Then the resulting total cost becomes “locked in.” If effects would change the total cost after this time, they have no effect.
Rule of thumb
Tax effects (Thalia, Sphere of Resistance & co.) are strong precisely AGAINST "free" spells: alternative costs don't protect from increases.
Stack
Force of Will
Alternative: exile a blue card + pay 1 life
Thalia, Guardian of Thraben
Noncreature spells cost more
The stack is empty.
Battlefield
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