Cheapest priced copy (Kaldheim): $0.49 at Card Kingdom, near mint, their price list as of .
Dragonkin Berserker 
Creature — Human Berserker · 2/2 · Rare · Kaldheim · #357
First strike
Boast abilities you activate cost {1} less to activate for each Dragon you control.
Boast — {4}{R}: Create a 5/5 red Dragon creature token with flying. (Activate only if this creature attacked this turn and only once each turn.)
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
5 printings · $0.49 to $0.79 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Kaldheim | 131 | Rare | $0.49 | $0.49 |
| Kaldheim | 357 | Rare | $0.59 | $0.49 |
| Kaldheim Promos | 131p | Rare | $0.49 | $0.49 |
| Kaldheim Promos | 131s | Rare | — | $0.79 |
| Starter Commander Decks | 134 | Rare | $0.49 | — |
6 rulings from Wizards of the Coast
- 5 February 2021Once you announce that you're activating a boast ability, no player may take actions until the ability has been paid for. Notably, opponents can't try to change the activation cost by removing your Dragons.
- 5 February 2021A boast ability can be activated at any point after the creature with that ability has been declared as an attacker. This can be before blockers are declared, after blockers are declared but before combat damage is dealt, during combat after combat damage is dealt, during the postcombat main phase, during the end step, or, in some unusual cases, during the cleanup step.
- 5 February 2021The cost reduction applies only to generic mana in the activation costs of boast abilities you activate. It can't reduce requirements of specific colors of mana.
- 5 February 2021If a creature with a boast ability is put onto the battlefield attacking, it was never declared as an attacker. Its boast ability can't be activated that turn.
- 5 February 2021If an effect adds additional combat phases to a turn and a creature with a boast ability attacks more than once during that turn, its boast ability can still be activated only once.
- 5 February 2021If it's not your turn and you gain control of a creature with a boast ability after that creature attacked, you can activate that creature's boast ability if it hasn't been activated yet that turn.