Cheapest priced copy (The Brothers' War Retro Artifacts): $109.99 at Card Kingdom, near mint, their price list as of .
Mox Amber 
Legendary Artifact · Mythic · Dominaria · #224
{T}: Add one mana of any color among legendary creatures and planeswalkers you control.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
6 printings · $109.99 to $549.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| The Brothers' War Retro Artifacts | 35 | Mythic | $109.99 | $279.99 |
| The Brothers' War Retro Artifacts | 98 | Mythic | $109.99 | $299.99 |
| The Brothers' War Retro Artifacts | 98z | Mythic | — | — |
| Dominaria | 224 | Mythic | $109.99 | $229.99 |
| The Hobbit Eternal | 96 | Mythic | $199.99 | $549.99 |
| Dominaria Promos | 224s | Mythic | — | $279.99 |
3 rulings from Wizards of the Coast
- 27 April 2018If you control no legendary creatures or legendary planeswalkers, you can activate Mox Amber's ability, but you won't add any mana.
- 27 April 2018Mox Amber's ability adds one mana of the color of your choice from among the colors of legendary creatures and legendary planeswalkers you control. It doesn't add one mana of each of those colors.
- 27 April 2018If your legendary creatures and legendary planeswalkers are all colorless, you can activate Mox Amber's ability, but you won't add any mana. Colorless is not a color.