Cheapest priced copy (Aetherdrift Commander): $4.49 at Card Kingdom, near mint, their price list as of .
Bontu's Monument 
Legendary Artifact · Uncommon · Aetherdrift Commander · #124
Black creature spells you cast cost {1} less to cast.
Whenever you cast a creature spell, each opponent loses 1 life and you gain 1 life.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
4 printings · $4.49 to $44.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Amonkhet | 225 | Uncommon | $5.99 | $44.99 |
| Dominaria United Commander | 179 | Uncommon | $4.99 | — |
| Aetherdrift Commander | 124 | Uncommon | $4.49 | — |
| Foundations Commander | 247 | Uncommon | $4.49 | — |
5 rulings from Wizards of the Coast
- 18 April 2017Each Monument has one ability that reduces the cost of creature spells of a certain color, and a triggered ability that triggers whenever you cast any creature spell—not just a creature spell of that color.
- 18 April 2017In a Two-Headed Giant game, the triggered ability of Bontu's Monument causes the opposing team to lose 2 life and you gain 1 life.
- 18 April 2017To determine the total cost of a creature spell, start with the mana cost or alternative cost you're paying, add any cost increases, then apply any cost reductions. The mana value of the creature remains unchanged, no matter what the total cost to cast it was.
- 18 April 2017Each Monument's triggered ability triggers as the creature spell is cast and resolves before that creature spell resolves. The ability will resolve even if that creature spell is countered.
- 18 April 2017A creature spell that's multiple colors is each of those colors. For example, Ahn-Crop Champion is a white creature and a green creature, so it can benefit from either Oketra's Monument or Rhonas's Monument—or both at once.