An object's mana cost is determined by the symbols in the upper right corner of the card, and the converted mana cost is derived solely from that. What you actually pay to cast a spell does not change that. The Spawn's converted mana cost is always 4.
Was that allowed, since the Spectacle was 3 mana, than its original 4 mana cost?
Former Rules Advisor
"Everything's better with pirates." - Lodge
(The Gamers: Dorkness Rising)
"Any sufficiently analyzed magic is indistinguishable from science."
(Girl Genius - Fairy Tale Theater Break - Cinderella, end of volume 8)