To determine the total cost of a spell, start with the mana cost or alternative cost you're paying, add any cost increases, then apply any cost reductions (such as that of Saheeli's second ability). The mana value of the spell remains unchanged, no matter what the total cost to cast it was.

The total cost to cast a spell is locked in before you pay that cost. For example, if you control three artifacts, including one you can sacrifice to add {C}, Saheeli's second ability will reduce the cost of a spell you cast by {3}. Then you can sacrifice the artifact when you activate mana abilities just before paying the cost, and it remains reduced by {3}.

Saheeli's second ability reduces only generic mana in the cost of the next spell you cast.

While resolving Saheeli's third ability, each token copies exactly what was printed on the original artifact and nothing else (unless that artifact is copying something else or is a token; see below). It doesn't copy whether that artifact is tapped or untapped, whether it has any counters on it or Auras attached to it, or any non-copy effects that have changed its types, color, or so on.

If a copied artifact has {X} in its mana cost, X is considered to be 0.

If a copied artifact is a token, the token that's created copies the original characteristics of that token as stated by the effect that created the token.

If a copied artifact is copying something else (for example, if the copied artifact is a Sculpting Steel), then the token enters the battlefield as whatever that artifact copied.

Any enters-the-battlefield abilities of a copied artifact will trigger when the token enters the battlefield. Any "as [this artifact] enters the battlefield" or "[this artifact] enters the battlefield with" abilities of the artifact will also work.