Like most Pokmon cards, the value of Mega Gengar ex 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 (Ascended Heroes)
2x Old Amber 2x Aerodactyl ex 2x Riolu 2x Lucario 2x Marshadow 2x Sabrina 2x Poke Ball 2x Professors Research 2x X Speed (or 1x X Speed and 1x Leaf) 2x Rocky Helmet Having two Lucario live on the bench means your Aerodactyl ex, Marshadow, and Lucario itself can deal 40 additional damage, which can easily make all the difference in a tough battle
For example, in the United States, the U.S
UR Ultra Rare Forbidden Forbidden Supreme King Dragon Starving Venom X DARK Level 8 [ DragonFusionEffect ] ATK 2800 DEF 2000 2 DARK Pendulum Monsters Must be either Fusion Summoned, or Special Summoned by Tributing the above cards you control (in which case you do not use "Polymerization")
Regardless, this assumption gives a roughly 92% chance of two or more hits, which is reasonable