As an additional cost to cast this spell, sacrifice an artifact or creature.
Draw cards equal to the mana value of the sacrificed permanent.
If a permanent has {X} in its mana cost, X is considered to be 0.
If an effect copies Morbid Curiosity, the controller of the copy will draw the same number of cards as determined by the original. Its controller can't and doesn't have to sacrifice a permanent.
To determine how many cards to draw, use the mana value of the permanent immediately before you sacrificed it.