Morbid Curiosity

• WWizards of the Coast2016-09-20
  Official Ruling
  If a permanent has in its mana cost, X is considered to be 0.
• WWizards of the Coast2016-09-20
  Official Ruling
  To determine how many cards to draw, use the mana value of the permanent immediately before you sacrificed it.
• WWizards of the Coast2016-09-20
  Official Ruling
  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.

Rulings via Scryfall