Cheapest priced copy (Modern Horizons 2): $0.49 at Card Kingdom, near mint, their price list as of .
Diamond Lion 
Artifact Creature — Cat · 2/2 · Rare · Modern Horizons 2 · #470
{T}, Discard your hand, Sacrifice this creature: Add three mana of any one color. Activate only as an instant.
Legal inCommanderModernLegacyVintage
Not legal inBrawlStandardPioneerPauper
4 printings · $0.49 to $1.29 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Modern Horizons 2 | 225 | Rare | $0.49 | $0.49 |
| Modern Horizons 2 | 427 | Rare | $0.79 | $0.79 |
| Modern Horizons 2 | 470 | Rare | $0.59 | $0.99 |
| Modern Horizons 2 Promos | 225s | Rare | — | $1.29Out of stock |
2 rulings from Wizards of the Coast
- 18 June 2021You can't cast an instant (or activate an ability as an instant) while casting a spell. Therefore, much like Lion's Eye Diamond, you can't activate the ability intending to use the mana to cast a spell from your hand.
- 18 June 2021Even though you can only activate this as an instant, it is still a mana ability. It doesn't use the stack and cannot be responded to.