If the copied Dragon has in its mana cost, X is 0.

If the Dragon that enters the battlefield is legendary and you have Sarkhan copy it, the two Dragons will have different names (unless the entering Dragon is somehow named Sarkhan, Soul Aflame). The "legend rule" won't apply.

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

If the copied Dragon is a token, Sarkhan copies the original characteristics of that token as stated by the effect that created that token, with the noted exceptions.

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

In some unusual cases, the Dragon that entered the battlefield will be copying something else by the time Sarkhan's ability resolves. In such a case, Sarkhan will copy whatever the Dragon is copying, with the noted exceptions.

Sarkhan copies exactly what was printed on the original Dragon and nothing else, with the noted exceptions (unless that creature is a token; see below). It doesn't copy whether that creature is tapped or untapped, whether it has any counters on it or Auras and Equipment attached to it, or any non-copy effects that have changed its power, toughness, types, color, and so on in response to Sarkhan's ability.