On FAQ1.7, we know that there is an additional cost formula “Do X to do Y”.

How about this effect "Action: Sacrifice a unit. If you do, gain 2 resources?" ?

Is "Sacrifice a unit" an additional cost before "If you do ...." sentence?

Thanks for reply...

No. For it to be part of the cost, it would need to explicitly be in the "do X to do Y" format - "Sacrifice a unit to gain 2 resources".

The most important difference is that since for the card you quote, Lash the Prisoner!, the sacrifice is not part of the cost, it doesn't happen until the effect actually resolves. So your opponent can respond with an action that removes the unit, and you won't get the 2 resources since you can't fulfill the "if you do" requirement. If it were "Sacrifice a unit to ..." the sacrifice would happen when you choose to trigger the effect and pay all costs, and your opponent couldn't stop it.

Wow, that is an extremely fine point in my mind, yet an important one nonetheless. "Pay all costs to play a card" would include the sacrifice in your "Sacrifice a unit to…" example, huh►

Yes. The "do X" part of a sentence in the "do X to do Y" format is always an additional cost. "Sacrifice a unit to …", "Corrupt a unit you control to …", "Sacrifice a development to …" … You do it when you trigger the effect and pay all costs. If you cannot do it, you cannot trigger the effect, so you for example need a unit in play you can corrupt or sacrifice when you trigger the effect, it would not be enough that one will be in play by the time the effect resolves. See "Triggering a card effect" in the FAQ.