Cheapest priced copy (Duskmourn: House of Horror Commander): $0.49 at Card Kingdom, near mint, their price list as of .
Nightmare Shepherd 
Enchantment Creature — Demon · 4/4 · Rare · Theros Beyond Death · #315
Flying
Whenever another nontoken creature you control dies, you may exile it. If you do, create a token that's a copy of that creature, except it's 1/1 and it's a Nightmare in addition to its other types.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
5 printings · $0.49 to $4.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Duskmourn: House of Horror Commander | 149 | Rare | $0.49 | — |
| Theros Beyond Death Promos | 108p | Rare | $0.59 | $0.79 |
| Theros Beyond Death Promos | 108s | Rare | — | $1.49 |
| Theros Beyond Death | 108 | Rare | $0.69 | $0.99 |
| Theros Beyond Death | 315 | Rare | $2.29 | $4.49 |
7 rulings from Wizards of the Coast
- 24 January 2020If the copied creature had {X} in its mana cost, X is considered to be 0.
- 24 January 2020If the copied creature was copying something else, then the token enters the battlefield as whatever that creature copied.
- 24 January 2020If the token is a copy of a creature whose power and toughness are defined by an ability (usually printed as */* or similar), the token doesn't copy the ability that defines its power and toughness. It remains a 1/1 creature.
- 24 January 2020The token copies exactly what was printed on the original creature and nothing else (unless that creature is copying something else; see below). It doesn't copy whether that creature was tapped or untapped, whether it had any counters on it or Auras or Equipment attached to it, or any non-copy effects that had changed its power, toughness, types, color, or so on.
- 24 January 2020The token copies the creature as it last existed on the battlefield before it died, not as it existed in the graveyard before it was exiled.
- 24 January 2020If something becomes a copy of the token, the copy is also a 1/1 and a Nightmare.
- 24 January 2020Any enters-the-battlefield abilities of the copied creature will trigger when the token enters the battlefield. Any "as [this creature] enters the battlefield" or "[this creature] enters the battlefield with" abilities of the creature will also work.