When assessing value, these elements can play an important role: Expansion and set: Some Pokemon expansions are simply more valuable than others
Karlach, Tiefling Guardian was first released in the set Alchemy Horizons: Baldur's Gate, illustrated by Billy Christian
I could also see it branching out beyond that into formats like Modern, however, where it has a lot to offer in Yawgmoth decks
a deck which offers minimal opportunity to interact or even play a normal game, but which still drains long minutes of their lives as the storm pilot plods through their interminable yet inevitable combo kill
One specific data point worth tracking: the Boa Hancock alternate-art parallel (around 20,000 JPY in Japan, stabilized) is scarce partly because of JP box shortages
The most important note I can leave you on is to always prioritize the goals of your sideboard