Actually you are copying , not casting and there is a diffrence. Hence there is a casting cost but 0 copying costs. For example:
You pay X to do X damage for a cost of a spell to cast it, copy does exactly that, copies so there is no payment. Only payment is the cost for the spell, which is that of copy. Better example: cast
lava axe for 5 mana, dealing 5 damage. Then cast
reverberate for 2 mana which makes a copy and deals 5 more damage. No additional cost of 5, making it 7 to get out the copy.
So again, casting and copying are 2 diffrent things.
Fling is another that comes to mind, you sacrifice a creature to cast, but pay nothing to copy it.