Edgar, Master Machinist

If an artifact you control has {X} in its mana cost, X is 0 for the purpose of calculating its mana value.

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

Edgar must be under your control in order to start casting an artifact spell using its first ability, but it doesn't need to stay under your control throughout that process or be under your control as the spell resolves. If, for example, you sacrifice Edgar to pay an additional cost of the spell or Edgar is destroyed in response, the spell will still resolve normally.