If I cast Howl of the Horde and then cast a Collateral Damage, I know that I do not need to sacrifice additional creatures for the copies based on this question. However, does each new copy trigger Prowess?
Collateral damage says:
As an additional cost to cast Collateral Damage ...
Comprehensive Rule 706.10 says:
To copy a spell ... means to put a copy of it onto the stack; a copy of a spell isn’t cast. ...
Per 706.10, the copy of Collateral Damage isn't cast, so you don't have to pay the cost again.
Copying a spell does not trigger Prowess. Prowess triggers when you cast a spell, and, as we previously discovered, copied spells are not cast. A ruling on Howl of the Horde reiterates what we have concluded:
... Each copy is created on the stack, so it’s not “cast.” Abilities that trigger when a player casts a spell won’t trigger. ...
Some cards actually copy the card (not the spell) and have you cast the copied card. Panoptic Mirror is a good example. Just to be clear, this would trigger Prowess.
From the card rulings:
9/20/2014: [...] Each copy is created on the stack, so it's not "cast". Abilities that trigger when a player casts a spell won't trigger.
Since the copy isn't "cast", you don't have to pay attention to any abilities saying "as an additional cost to cast [card]...". This also means that prowess does not trigger.