Cheapest priced copy (March of the Machine): $0.59 at Card Kingdom, near mint, their price list as of .
Boon-Bringer Valkyrie 
Creature — Angel Warrior · 4/4 · Rare · March of the Machine · #343
Backup 1 (When this creature enters, put a +1/+1 counter on target creature. If that's another creature, it gains the following abilities until end of turn.)
Flying, first strike, lifelink
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
4 printings · $0.59 to $1.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| March of the Machine | 9 | Rare | $0.59 | $0.79 |
| March of the Machine | 343 | Rare | $0.59 | $0.79 |
| March of the Machine Promos | 9p | Rare | $1.29 | $0.79 |
| March of the Machine Promos | 9s | Rare | — | $1.49 |
5 rulings from Wizards of the Coast
- 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 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 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 backup ability causes another creature to gain abilities, the creature with backup will still have those abilities.