Because this card was awarded as a prize and there are so few in existence, the $20k - $30k value seems well-justified
Theres a lot of ways to double up on the lifegain, and this is the cheapest, in addition to the cost reduction for everything else youre doing
If you re-ban Golgari Grave-Troll, the deck simply slots in Golgari Thug, and while the deck is slightly weaker since it will dredge fewer cards on average, there's a good chance that, with the amount of support the archetype has, Dredge would still remain one of the best decks in the format
Its the only ability of its type in Pokemon TCG Pocket and even if youre not planning to keep your Garchomp in the active slot, it can be handy for drawing into your Engine
Legality depends on the tournament format -- Fiendsmith's Sanct can be legal in some formats and banned or restricted in others