Cheapest priced copy (Adventures in the Forgotten Realms): $0.69 at Card Kingdom, near mint, their price list as of .
Hand of Vecna 
Legendary Artifact — Equipment · Rare · Adventures in the Forgotten Realms · #246
At the beginning of combat on your turn, equipped creature or a creature you control named Vecna gets +X/+X until end of turn, where X is the number of cards in your hand.
Equip—Pay 1 life for each card in your hand.
Equip {2}
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
5 printings · $0.69 to $12.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Adventures in the Forgotten Realms | 246 | Rare | $0.69 | $0.79 |
| Adventures in the Forgotten Realms | 394 | Rare | $0.69 | $0.79 |
| Adventures in the Forgotten Realms Promos | 246a | Rare | — | $12.99 |
| Adventures in the Forgotten Realms Promos | 246p | Rare | $1.29 | $3.49 |
| Adventures in the Forgotten Realms Promos | 246s | Rare | — | $1.79 |
2 rulings from Wizards of the Coast
- 23 July 2021If you control both Vecna and Hand of Vecna equipped to another creature, you choose which creature to apply the bonus to as the triggered ability resolves.
- 23 July 2021Hand of Vecna's triggered ability uses the number of cards in your hand as it resolves to determine the bonus the creature gets. It won't change if the number of cards in your hand changes later in the turn.