Cheapest priced copy (March of the Machine Commander): $3.99 at Card Kingdom, near mint, their price list as of .
Emergent Woodwurm 
Creature — Wurm · 4/4 · Rare · March of the Machine Commander · #124
Backup 3 (When this creature enters, put three +1/+1 counters on target creature. If that's another creature, it gains the following ability until end of turn.)
Whenever this creature attacks, look at the top X cards of your library, where X is its power. You may put a permanent card with mana value X or less from among them onto the battlefield. Put the rest on the bottom of your library in a random order.
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
2 printings · $3.99 to $4.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| March of the Machine Commander | 37 | Rare | $4.49 | — |
| March of the Machine Commander | 124 | Rare | $3.99 | — |
6 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 backup ability targets the creature with backup, that creature will get +1/+1 counters, but it won’t gain additional abilities.
- 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 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 2023Use the power of the creature as the triggered ability resolves to determine the value of X. If the creature is no longer on the battlefield at that time, use its power from when it was last on the battlefield.
- 14 April 2023If a backup ability causes another creature to gain abilities, the creature with backup will still have those abilities.