Like most Magic: The Gathering cards, the value of Unstable Mutation 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 (Foreign Black Border)
You also arent benefitting the most from oozes that mimic other creatures abilities or that base their power and toughness on your other creatures MV, P/T, etc
If you have multiple Barnes & Nobles in your area, I highly recommend checking them all out to find out which one has the best selection
The set and the block came right after the Urza block (Urzas Saga, Urzas Legacy, Urzas Destiny), which broke standard in all sorts of ways and resulted in numerous bannings
There are only 10 of them out there, making the going price for one well near $1,000
Who knew I needed a Spider-Rex card this bad