Cheapest priced copy (Commander 2021): $0.49 at Card Kingdom, near mint, their price list as of .
Erratic Cyclops 
Creature — Cyclops Shaman · 0/8 · Rare · Guilds of Ravnica · #98
Trample
Whenever you cast an instant or sorcery spell, this creature gets +X/+0 until end of turn, where X is that spell's mana value.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
2 printings · $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Commander 2021 | 166 | Rare | $0.49 | — |
| Guilds of Ravnica | 98 | Rare | $0.49 | $0.49 |
2 rulings from Wizards of the Coast
- 5 October 2018If a spell has {X} in its mana cost, include the value chosen for that X when determining the mana value of that spell.
- 5 October 2018Erratic Cyclops’s triggered ability resolves before the spell that caused it to trigger. It resolves even if that spell is countered.