Cheapest priced copy (Innistrad): $1.29 at Card Kingdom, near mint, their price list as of .
Moonmist 
Instant · Common · The List · #ISD-195
Transform all Humans. Prevent all combat damage that would be dealt this turn by creatures other than Werewolves and Wolves. (Only double-faced cards can be transformed.)
Legal inCommanderModernLegacyVintagePauper
Not legal inBrawlStandardPioneer
2 printings · $1.29 to $39.99 at Card Kingdom
3 rulings from Wizards of the Coast
- 22 September 2011Whether or not a creature is a Werewolf or a Wolf is checked only as combat damage is dealt. If the creature isn’t a Werewolf or a Wolf at that time, its combat damage will be prevented.
- 22 September 2011Moonmist causes any double-faced Human to transform, not just Werewolves.
- 22 September 2011Moonmist will prevent combat damage dealt by a creature that isn’t a Werewolf or a Wolf even if that creature wasn’t on the battlefield (or was a Werewolf or a Wolf) when Moonmist resolved.