Like most Pokmon cards, the value of Aggron 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 (Mysterious Treasures)
I did manage to win a few hundred dollars and a large hammock that I kept in my apartment during my bachelor days
Theres a few ways you can grind out a win from such a dominant defensive posture: you can chip away their life with your Ring-bearer or an Eagle air force, or even piling equipment onto one token at a time and forcing advantageous trades
Once Rust-Shield Rampager has been blocked, reducing the power of a blocking creature to 2 or less won't remove that creature from combat or cause Rust-Shield Rampager to become unblocked
For a beginner, this can become overwhelming to manage, and this is on top of a 4-player, multiplayer Battlefield
These aren't the only appearances of these characters, however