Cheapest priced copy (Dominaria United Commander): $1.49 at Card Kingdom, near mint, their price list as of .
The Mana Rig 
Legendary Artifact · Mythic · Dominaria United Commander · #80
Whenever you cast a multicolored spell, create a tapped Powerstone token. (It's an artifact with "{T}: Add {C}. This mana can't be spent to cast a nonartifact spell.")
{X}{X}{X}, {T}: Look at the top X cards of your library. Put up to two of them into your hand and the rest on the bottom of your library in a random order.
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
2 printings · $0.99 to $1.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Dominaria United Commander | 28 | Mythic | $1.49 | $1.99 |
| Dominaria United Commander | 80 | Mythic | $1.49 | $0.99 |
3 rulings from Wizards of the Coast
- 9 September 2022You can use the {C} added by a Powerstone token on anything that isn’t a nonartifact spell. This includes paying costs to activate abilities of both artifact and nonartifact permanents, paying ward costs, and so on.
- 9 September 2022Powerstone tokens are a new kind of predefined token. Each one has the artifact subtype “Powerstone” and the ability “{T}: Add {C}. This mana can’t be spent to cast a nonartifact spell.”
- 9 September 2022Although all the cards in the Dominaria United set that create Powerstone tokens create a tapped Powerstone token, entering the battlefield tapped isn’t part of the token’s definition. Notably, if you create a token that is a copy of a Powerstone token, the token copy won’t enter the battlefield tapped.