Cheapest priced copy (Mystery Booster 2): $0.49 at Card Kingdom, near mint, their price list as of .
Hollow One 
Artifact Creature — Golem · 4/4 · Rare · Store Championships · #25
This spell costs {2} less to cast for each card you've cycled or discarded this turn.
Cycling {2} ({2}, Discard this card: Draw a card.)
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
5 printings · $0.49 to $44.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Hour of Devastation | 163 | Rare | $0.69 | $2.49 |
| Mystery Booster 2 | 95 | Rare | $0.49 | — |
| Hour of Devastation Promos | 163s | Rare | — | $4.49Out of stock |
| Store Championships | 25 | Rare | — | $3.49 |
| Time Spiral Remastered | 396 | Special | $1.79 | $44.99 |
4 rulings from Wizards of the Coast
- 19 March 2021Once you've discarded three cards during one turn, Hollow One costs {0} to cast. It won't stop at {1} or cost negative amounts of mana.
- 19 March 2021Hollow One's first ability doesn't give you permission to discard cards. You'll need another effect that instructs or allows you to discard them, such as a cycling ability.
- 19 March 2021To determine the total cost of a spell, start with the mana cost or alternative cost you're paying, add any cost increases, then apply any cost reductions (such as that of Hollow One). The mana value of the spell is determined by only its mana cost, no matter what the total cost to cast that spell was.
- 19 March 2021Hollow One's cost is reduced even if the cards you've cycled or discarded aren't in your graveyard.