Cheapest priced copy (Guilds of Ravnica): $0.49 at Card Kingdom, near mint, their price list as of .
Chamber Sentry 
Artifact Creature — Construct · 0/0 · Rare · Guilds of Ravnica · #232
This creature enters with a +1/+1 counter on it for each color of mana spent to cast it.
{X}, {T}, Remove X +1/+1 counters from this creature: It deals X damage to any target.
{W}{U}{B}{R}{G}: Return this card from your graveyard to your hand.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
2 printings · $0.49 to $0.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Guilds of Ravnica | 232 | Rare | $0.49 | $0.99 |
| Jumpstart | 461 | Rare | $0.49 | — |
6 rulings from Wizards of the Coast
- 5 October 2018You can choose any value for X as you cast Chamber Sentry. The value chosen for X doesn't directly affect the number of +1/+1 counters Chamber Sentry enters the battlefield with, but it does let you pay more mana and thus spend more colors of mana to cast it.
- 5 October 2018If Chamber Sentry enters the battlefield without being cast, it receives no +1/+1 counters.
- 5 October 2018If an effect allows you to spend mana as though it were mana of any color to cast Chamber Sentry, you consider only the actual mana spent to determine how many counters it receives.
- 5 October 2018If another creature enters the battlefield as a copy of Chamber Sentry, consider what mana was spent to cast that creature to determine how many +1/+1 counters it enters with.
- 5 October 2018Colorless mana won't give Chamber Sentry another +1/+1 counter. Colorless is not a color.
- 5 October 2018The value of X chosen when you activate Chamber Sentry's first activated ability doesn't have to be the same value of X that you chose when you cast it.