Cheapest priced copy (Commander Masters): $1.49 at Card Kingdom, near mint, their price list as of .
Ancient Stone Idol 
Artifact Creature — Golem · 12/12 · Rare · Murders at Karlov Manor Commander · #222
Flash
This spell costs {1} less to cast for each attacking creature.
Trample
When this creature dies, create a 6/12 colorless Construct artifact creature token with trample.
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
4 printings · $1.49 to $1.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Commander 2018 | 53 | Rare | $1.79 | — |
| Commander Masters | 940 | Rare | $1.49 | — |
| Murders at Karlov Manor Commander | 222 | Rare | $1.79 | — |
| March of the Machine Commander | 347 | Rare | $1.99 | — |
3 rulings from Wizards of the Coast
- 13 July 2018If there are enough attacking creatures, Ancient Stone Idol's cost can be reduced to {0}.
- 13 July 2018The total cost to cast a spell is locked in before you pay that cost. For example, if you control five attacking creatures, including one you can sacrifice to add {C}, Ancient Stone Idol costs {5} to cast. Then you can sacrifice the creature when you activate mana abilities just before paying the cost, and it still costs only {5} to cast.
- 13 July 2018Ancient Stone Idol's cost is reduced for each attacking creature, not just creatures attacking you. It even counts your attacking creatures.