Like most Magic: The Gathering cards, the value of Geralf's Masterpiece depends on a few consistent factors: its rarity, the printing and finish (regular or foil), its physical condition, and the set it was printed in (Shadows over Innistrad)
Its essential for card resellers to keep their cards in mint condition
If you are the one attacking, then keep track of how much HP your opponent still has
Sure, you have to invest some mana and probably wait a turn, but you can make some sweet deals to get rid of problematic permanents
Then, you'll trigger Promethean Princess, Bestower of Flames, and she will destroy your random Fire-attribute monster and one of your opponent's monsters to summon herself from your graveyard (remember to put her in play co-linked to "Zealantis")