Like most Pokmon cards, the value of Meloetta 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 (XY Black Star Promos)
When you play it, your hand will fill back up to seven cards, but your opponents hand size wont have changed only its contents will be different
Vampire Nocturnus gives such a strong boost to your team of Vampires (itself included)

A Little Chat Abigale, Poet Laureate Aboleth Spawn Absorbing Man Abyssal Harvester Activated Sleeper Adagia, Windswept Bastion Adventurous Eater Aethertow Agate Instigator Agrus Kos, Eternal Soldier Altar Ibn-La'Ahad Altered Ego Amphibian Downpour Amphin Mutineer Angel of Indemnity Angel of Sanctions Anhelo, the Painter Anikthea, Hand of Erebos Anointer Priest Applied Geometry Araumi of the Dead Tide Arboreal Alliance Arcane Artisan Arcane Heist Arcane Proxy Arcane Savant Ardyn, the Usurper Arna Kennerd, Skycaptain Artisan of Forms Ashad, the Lone Cyberman Ashling's Command Ashling, Flame Dancer Ashnod the Uncaring Assemble from Parts Assembly Hall Assimilation Aegis Astral Steel Audacious Swap Aurora Shifter Auton Soldier Aven Wind Guide Back from the Brink Banish into Fable Banshee of the Dread Choir Barkshell Blessing Baron Helmut Zemo Barroom Brawl Battle for Bretagard Battlemage's Bracers Beamsplitter Mage Belonging Benthic Anomaly Bifurcate Biowaste Blob Blade of Selves Blade of Shared Souls Blazing Firesinger Bloodforged Battle-Axe Bloodthirsty Adversary Blue Mage's Cane Blue Ribbon Blue Sun's Twilight Body Double Bramble Sovereign Brass Knuckles Brenard, Ginger Sculptor Briarblade Adept Brigid's Command Broodbirth Viper Brudiclad, Telchor Engineer Burn Trail

Sun & Moon's trademark Tag Team Pokemon and popular Trainer cards are hallmarks of the most valuable Sun & Moon sets
Above the $200 threshold, the grading-fee delta is smaller than the grade-risk discount, so paying the PSA 10 premium upfront is usually correct