Cheapest priced copy (Commander 2021): $0.49 at Card Kingdom, near mint, their price list as of .
Hydra Broodmaster 
Creature — Hydra · 7/7 · Rare · Modern Horizons 3 Commander · #233
{X}{X}{G}: Monstrosity X. (If this creature isn't monstrous, put X +1/+1 counters on it and it becomes monstrous.)
When this creature becomes monstrous, create X X/X green Hydra creature tokens.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
4 printings · $0.49 to $1.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Commander 2021 | 194 | Rare | $0.49 | — |
| Magic 2015 Clash Pack | 4 | Rare | — | $0.69 |
| Journey into Nyx | 128 | Rare | $0.59 | $1.49 |
| Modern Horizons 3 Commander | 233 | Rare | $0.49 | $0.49 |
4 rulings from Wizards of the Coast
- 26 April 2014The value of each X in Hydra Broodmaster's last ability is equal to the value chosen for X when its monstrosity ability was activated.
- 26 April 2014An ability that triggers when a creature becomes monstrous won't trigger if that creature isn't on the battlefield when its monstrosity ability resolves.
- 26 April 2014Monstrous isn't an ability that a creature has. It's just something true about that creature. If the creature stops being a creature or loses its abilities, it will continue to be monstrous.
- 26 April 2014Once a creature becomes monstrous, it can't become monstrous again. If the creature is already monstrous when the monstrosity ability resolves, nothing happens.