I think that I get what this card was meant to do. In which case the oracle text is messed up because the original text says "which you still have to spend" in regards to the card's casting cost.

Here is my interpretation of what this card should say:

Variable Colorless:Play a creature spell face down as a 2/2 creature for Variable Colorless. Variable Colorless can be any part of that creature's casting cost, including 0. Colors in the casting cost may not be paid with inappropriate mana.
If a creature played face down with illusionary mask would deal or receive damage or be tapped, turn it face up and pay the remainder of the casting cost or sacrifice it.

My interpretation of what this card is meant to do:

Let's say you are playing green. You have this mask. You have three green mana available. You play a Sedge Scorpion and two Leatherback Baloths as face-down 2/2's all for 0. Next turn you have four green available and you state that you are attacking with a face down pair ... (see all)