Like most Pokmon cards, the value of Yanmega 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 (POP Series 8)
Its unclear how many Holofoil variants Topsun printed, although collectors have documented up to 16 variations, including: Mewtwo / Venusaur Zapdos / Aerodactyl Jigglypuff / Eevee Venusaur / Electabuzz Clefairy / Horsea Farfetchd / Hitmonchan Meowth / Cubone Vulpix / Hypno Dragonite / Mr
However, the frontside is the powerful one
The Wilds Fun: 1 Impact: 1 Chaos: 3 This is on the same level as Ten Wizards Mountain
You should also keep in mind that new Pokmon card releases may not have these dates
As they are all creatures, and lack rules text, their function remains largely understandable even to players who are unable to read them