Cheapest priced copy (Guilds of Ravnica): $2.49 at Card Kingdom, near mint, their price list as of .
Runaway Steam-Kin 
Creature — Elemental · 1/1 · Rare · Guilds of Ravnica · #115
Whenever you cast a red spell, if this creature has fewer than three +1/+1 counters on it, put a +1/+1 counter on this creature.
Remove three +1/+1 counters from this creature: Add {R}{R}{R}.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
3 printings · $2.49 to $15.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Guilds of Ravnica | 115 | Rare | $2.49 | $12.99 |
| Guilds of Ravnica Promos | 115p | Rare | $4.49 | $15.99 |
| Guilds of Ravnica Promos | 115s | Rare | — | $13.99 |
3 rulings from Wizards of the Coast
- 5 October 2018Runaway Steam-Kin’s triggered ability resolves before the spell that caused it to trigger. It resolves even if that spell is countered.
- 5 October 2018If Runaway Steam-Kin has three or more +1/+1 counters on it after you’re done paying for a red spell, its ability doesn’t trigger at all. If it has three or more +1/+1 counters on it as its triggered ability resolves, it doesn’t get a +1/+1 counter.
- 5 October 2018Runaway Steam-Kin has to be on the battlefield for its ability to trigger. Casting it won’t trigger its own ability.