Cheapest priced copy (Crimson Vow Commander): $0.49 at Card Kingdom, near mint, their price list as of .
Spectral Arcanist 
Creature — Spirit Wizard · 3/2 · Rare · Crimson Vow Commander · #53
Flying
When this creature enters, you may cast an instant or sorcery spell with mana value less than or equal to the number of Spirits you control from a graveyard without paying its mana cost. If that spell would be put into a graveyard, exile it instead.
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
2 printings · $0.49 to $0.69 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Crimson Vow Commander | 15 | Rare | $0.49 | — |
| Crimson Vow Commander | 53 | Rare | $0.69 | — |
3 rulings from Wizards of the Coast
- 19 November 2021If you choose to cast the spell, you must do so as Spectral Arcanist's ability is resolving. You can't wait and cast it later.
- 19 November 2021If you cast a spell with X in its mana cost this way, you must choose 0 as the value for X.
- 19 November 2021If you cast a spell without paying its mana cost, you may choose to pay additional costs (such as kicker) but not alternative costs (such as flashback).