Cheapest priced copy (Dominaria United): $0.49 at Card Kingdom, near mint, their price list as of .
Radha's Firebrand 
Creature — Human Warrior · 3/1 · Rare · Dominaria United · #408
Whenever this creature attacks, target creature defending player controls with power less than this creature's power can't block this turn.
Domain — {5}{R}: This creature gets +2/+2 until end of turn. This ability costs {1} less to activate for each basic land type among lands you control. Activate only once each turn.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
4 printings · $0.49 to $0.79 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Dominaria United | 141 | Rare | $0.49 | $0.49 |
| Dominaria United | 408 | Rare | $0.49 | $0.49 |
| Dominaria United Promos | 141p | Rare | $0.49 | $0.49 |
| Dominaria United Promos | 141s | Rare | — | $0.79 |
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 2022Increasing the target creature's power in response to Radha's Firebrand's first ability can cause it to be an illegal target for the ability. However, once the ability has resolved, it won't be able to block even if its power becomes greater than Radha's Firebrand's power.
- 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.