Cheapest priced copy (Foundations Jumpstart): $0.59 at Card Kingdom, near mint, their price list as of .
Dreadhorde Arcanist 
Creature — Zombie Wizard · 1/3 · Special · Time Spiral Remastered · #341
Trample
Whenever this creature attacks, you may cast target instant or sorcery card with mana value less than or equal to this creature's power from your graveyard without paying its mana cost. If that spell would be put into your graveyard, exile it instead.
Legal inCommanderPioneerModernVintage
Banned inLegacy
Not legal inBrawlStandardPauper
5 printings · $0.59 to $34.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Foundations Jumpstart | 539 | Rare | $0.59 | — |
| War of the Spark Promos | 125p | Rare | $0.59 | $3.49 |
| War of the Spark Promos | 125s | Rare | — | $2.99 |
| Time Spiral Remastered | 341 | Special | $1.29 | $34.99 |
| War of the Spark | 125 | Rare | $0.59 | $3.49 |
5 rulings from Wizards of the Coast
- 19 March 2021The instant or sorcery card you target must have mana value less than or equal to Dreadhorde Arcanist's power immediately after Dreadhorde Arcanist has attacked. Any other abilities that trigger when creatures you control attack won't have resolved yet.
- 19 March 2021If the card has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.
- 19 March 2021If Dreadhorde Arcanist leaves the battlefield before its triggered ability resolves, use its power as it last existed on the battlefield to check whether the card is still a legal target.
- 19 March 2021If you cast a card "without paying its mana cost," you can't choose to cast it for any alternative costs. You can, however, pay additional costs. If the card has any mandatory additional costs, you must pay those to cast the card.
- 19 March 2021If you cast the card, you do so as part of the resolution of Dreadhorde Arcanist's triggered ability. You can't wait to cast it later in the turn. Timing permissions based on a card's type are ignored, and the spell will resolve before blockers are declared.