Like most Magic: The Gathering cards, the value of Mammoth Umbra 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 (Rise of the Eldrazi)
If you're curious, Google "binomial coefficient," which is the name of those vertical parentheses with numbers
So, if you dont like what comes next, you can hope for a better hit by using this
Weakness: Darkness (x2) Resistance: Fighting (-30) Retreat: 1 Meowstic - Psychic - HP100 Stage 1 - Evolves from Espurr [P][C] Trick Step: 80 damage
Buying from an actual retailer cuts out some marketplace risks
Chart shows the price of Rattata [Reverse Holo] #66 at the end of each month going back as long as we have tracked the item.