The value of X is calculated only once, as Doran's last ability resolves.

Doran's first ability applies only to generic mana in the total cost of creature spells you cast with toughness greater than their power.

To determine the total cost of a spell, start with the mana cost or alternative cost you're paying (such as a flashback cost), add any cost increases (such as kicker costs), then apply any cost reductions (such as that of this ability). The mana value of the spell is determined by only its mana cost, no matter what the total cost to cast that spell was.

To find the difference between a creature's power and its toughness, subtract the smaller of those two numbers from the larger one. For example, the difference between the power and toughness of a 3/5 creature is 2. The difference between the power and toughness of a 5/3 creature is also 2.