The value of X is determined only once, at the time Rowan, Scion of War's activated ability resolves.
Rowan's activated ability doesn't change the mana cost or mana value of any spell. It changes only the total cost you pay.
Rowan'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.
Rowan's activated ability counts the total amount of life you lost without taking into account any life you gained during that turn. For example, if you gained 3 life and lost 3 life earlier in the turn, the cost of black and/or red spells you cast this turn will be reduced by 
.
: Spells you cast this turn that are black and/or red cost 
less to cast, where X is the amount of life you lost this turn. Activate only as a sorcery.
