Cheapest priced copy (Dragon's Maze): $0.69 at Card Kingdom, near mint, their price list as of .
Riot Control 
Instant · Common · Dragon's Maze · #6
You gain 1 life for each creature your opponents control. Prevent all damage that would be dealt to you this turn.
Legal inCommanderPioneerModernLegacyVintagePauper
Not legal inBrawlStandard
2 printings · $0.69 to $4.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Bloomburrow Commander | 151 | Common | $0.79 | — |
| Dragon's Maze | 6 | Common | $0.69 | $4.99 |
2 rulings from Wizards of the Coast
- 15 April 2013Riot Control doesn't prevent combat damage that would be dealt to planeswalkers you control. It also doesn't prevent damage that would be dealt to creatures you control.
- 15 April 2013Count the number of creatures your opponents control as Riot Control resolves to determine how much life you gain.