In the rules supplement it gives steps for revealing an enchantment. Step three is to pay costs. Rather than worrying about the value of zero, you can think of this rule as saying that step is optional and opting to skip the pay costs step results in the spell being discarded without effect, regardless of the cost or lack thereof. You are choosing to not do this step before the game even looks at the cost, because that step is where it would have looked at it.
