Cheapest priced copy (Commander 2021): $0.49 at Card Kingdom, near mint, their price list as of .
Fractal Harness 
Artifact — Equipment · Rare · Secrets of Strixhaven Commander · #268
When this Equipment enters, create a 0/0 green and blue Fractal creature token. Put X +1/+1 counters on it and attach this Equipment to it.
Whenever equipped creature attacks, double the number of +1/+1 counters on it.
Equip {2}
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
3 printings · $0.49 to $2.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Commander 2021 | 61 | Rare | $0.49 | — |
| Commander 2021 | 388 | Rare | $2.49 | — |
| Secrets of Strixhaven Commander | 268 | Rare | $0.49 | — |
2 rulings from Wizards of the Coast
- 16 April 2021If you put Fractal Harness onto the battlefield without casting it, or if you choose 0 as the value of X, the token you create will be 0/0. Unless something else is raising its toughness, that token will enter the battlefield and die before you could take any actions.
- 16 April 2021To double the number of +1/+1 counters on a permanent, put a number of +1/+1 counters on it equal to the number it already has. Other cards that interact with putting counters on it will interact with this effect accordingly.