The mana value of a spell cast without paying its mana cost is determined by its mana cost, even though that cost wasn't paid.
Sibylline Soothsayer
Exiling a card with suspend isn't casting that card. This action doesn't use the stack and can't be responded to.
You can exile a card in your hand using suspend any time you could cast that card. Consider its card type, any effects that modify when you could cast it (such as flash) and any other effects that stop you from casting it (such as from Meddling Mage's ability) to determine if and when you can do this. Whether you could actually complete all steps in casting the card is irrelevant. For example, you can exile a card with suspend that has no mana cost or that requires a target even if no legal targets are available at that time.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Anna Pavleeva
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