If a card in your graveyard has {X} in its mana cost, X is 0 for the purpose of determining its mana value.

If an effect allows you to cast Astelli Reclaimer without paying its mana cost, X will be 0.

If an effect requires you to pay some amount of mana to prevent Astelli Reclaimer from being countered, as Divert Disaster does, that mana wasn’t spent to cast Astelli Reclaimer and will not be taken into account.

If a copy of Astelli Reclaimer is created on the stack, no mana was spent to cast the copy, so X will be 0. The amount of mana spent to cast the original spell is not copied.

Astelli Reclaimer’s enters ability cares about the mana you actually paid to cast Astelli Reclaimer, not its mana cost. If you cast Astelli Reclaimer for {3}{W}{W}, X is 5. If you cast it with warp for {2}{W}, X is 3. Any effects that increase or decrease the cost to cast it will also be taken into account.