Cheapest priced copy (March of the Machine): $0.35 at Card Kingdom, near mint, their price list as of .
Mirror-Shield Hoplite 
Creature — Human Soldier · 2/2 · Uncommon · March of the Machine · #247
Vigilance
Whenever a creature you control becomes the target of a backup ability, copy that ability. You may choose new targets for the copy. This ability triggers only once each turn.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
1 printing · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| March of the Machine | 247 | Uncommon | $0.35 | $0.49 |
7 rulings from Wizards of the Coast
- 14 April 2023The abilities that backup grants to the target creature are determined only once, at the time the ability triggers. They won’t change if the permanent with backup loses any abilities before the backup ability resolves.
- 14 April 2023If a permanent enters the battlefield as a copy of a card with a backup ability or a token is created that is a copy of that card, the order of the printed abilities is maintained.
- 14 April 2023Backup confers only abilities that are actually printed below it. Any abilities that are gained by the permanent are ignored, including abilities gained due to a resolving spell or ability or copy effects.
- 14 April 2023If a backup ability targets the creature with backup, that creature will get +1/+1 counters, but it won’t gain additional abilities.
- 14 April 2023If a creature other than the creature with backup ends up as the target of both the backup ability and the copy, it will end up with two +1/+1 counters and it will gain the non-backup abilities twice. In some cases, the duplicate abilities will be redundant (two instances of flying, for example). However, if it picks up two triggered abilities, for example, those abilities trigger independently.
- 14 April 2023The source of the copy is the same as the source of the original backup ability. If the creature that ends up as the target of the copy is that original creature with backup, it will get a +1/+1 counter, but it won’t gain any abilities. If it’s a creature other than that original creature, it will get a +1/+1 counter and gain abilities.
- 14 April 2023If a backup ability causes another creature to gain abilities, the creature with backup will still have those abilities.