Cheapest priced copy (Dominaria United): $0.59 at Card Kingdom, near mint, their price list as of .
Briar Hydra 
Creature — Plant Hydra · 6/6 · Rare · Dominaria United · #286
Trample
Domain — Whenever this creature deals combat damage to a player, put X +1/+1 counters on target creature you control, where X is the number of basic land types among lands you control.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
2 printings · $0.59 to $1.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Dominaria United | 286 | Rare | $0.59 | $1.99 |
| Dominaria United | 427 | Rare | $0.69 | — |
3 rulings from Wizards of the Coast
- 9 September 2022The basic land types are Plains, Island, Swamp, Mountain, and Forest. Land types other than basic land types (such as Desert) don’t contribute to domain abilities.
- 9 September 2022The value of X is determined as Briar Hydra’s last ability resolves.
- 9 September 2022Domain abilities count the number of basic land types among lands you control, not how many lands you control or how many of any type.