Cheapest priced copy (Final Fantasy Commander): $7.49 at Card Kingdom, near mint, their price list as of .
Walking Ballista 
Artifact Creature — Construct · 0/0 · Rare · Mystery Booster 2 · #238
This creature enters with X +1/+1 counters on it.
{4}: Put a +1/+1 counter on this creature.
Remove a +1/+1 counter from this creature: It deals 1 damage to any target.
Legal inCommanderModernLegacyVintage
Banned inPioneer
Not legal inBrawlStandardPauper
10 printings · $7.49 to $109.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Double Masters | 306 | Rare | $10.99 | $19.99 |
| Aether Revolt | 181 | Rare | $9.49 | $27.99 |
| Final Fantasy Commander | 371 | Rare | $7.49 | $13.99 |
| Jumpstart 2022 | 806 | Rare | $9.49 | — |
| Mystery Booster 2 | 238 | Rare | $11.99 | $69.99 |
| Aether Revolt Promos | 181s | Rare | — | $32.99 |
| Fallout (Assaultron Invader) | 352 | Rare | $32.99 | $29.99 |
| Fallout (Assaultron Invader) | 880 | Rare | — | $109.99 |
| Secret Lair Drop | 58 | Rare | — | $47.99 |
| Secret Lair Drop | 1265 | Rare | — | $84.99 |
2 rulings from Wizards of the Coast
- 7 August 2020If Walking Ballista has been dealt damage or had its toughness reduced by an effect, this limits how many times you'll be able to remove +1/+1 counters from it in a single turn. For example, if it has three +1/+1 counters on it and has been dealt 1 damage this turn, it will be destroyed immediately after you activate the ability a second time and you won't be able to activate it a third time.
- 7 August 2020A casting cost of {X}{X} means that you pay twice X. If you want X to be 3, you pay {6} to cast Walking Ballista.