The value of X is determined only once, at the time Will, Scion of Peace's activated ability resolves.

Will's activated ability doesn't change the mana cost or mana value of any spell. It changes only the total cost you pay.

Will's activated ability can't reduce the amount of colored mana you pay for a spell. It reduces only the generic mana component of that cost.

Will's activated ability counts the total amount of life you gained without taking into account any life you lost during that turn. For example, if you gained 3 life and lost 3 life earlier in the turn, the cost of white and/or blue spells you cast this turn will be reduced by {3}.