For example: I have Urza's Incubator on the table, and I declare Spirits as my creature type.
I then want to cast Mirrorhall Mimic (A spirit creature) from my graveyard for its Disturb cost which reads: "You may cast this card from your graveyard transformed for its Disturb cost". In isolation, this would have it's spell cost reduced.
What enters the battlefield however, is an Enchantment called Ghastly Mimicry, which would not be impacted by the spell cost reduction.
Rule 712.7 states:
If a transforming double-faced card is cast as a spell, it’s put on the stack with its front face up by default. If a transforming double-faced card is cast “transformed,” it’s put on the stack with its back face up.
What I don't fully understand is this: The enchantment doesn't have a casting cost printed on it, so am I or am I not casting a Spirit creature that then transforms in to an enchantment and who's cost is reduced by the effect of Urza's Incubator.