Cheapest priced copy (Ravnica Allegiance Promos): $0.49 at Card Kingdom, near mint, their price list as of .
Growth-Chamber Guardian 
Creature — Elf Crab Warrior · 2/2 · Rare · Ravnica Allegiance · #128
{2}{G}: Adapt 2. (If this creature has no +1/+1 counters on it, put two +1/+1 counters on it.)
Whenever one or more +1/+1 counters are put on this creature, you may search your library for a card named Growth-Chamber Guardian, reveal it, put it into your hand, then shuffle.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
3 printings · $0.49 to $1.29 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Ravnica Allegiance Promos | 128p | Rare | $0.49 | $0.79 |
| Ravnica Allegiance Promos | 128s | Rare | — | $1.29 |
| Ravnica Allegiance | 128 | Rare | $0.49 | $0.49 |
3 rulings from Wizards of the Coast
- 25 January 2019You can always activate an ability that will cause a creature to adapt. As that ability resolves, if the creature has a +1/+1 counter on it for any reason, you simply won't put any +1/+1 counters on it.
- 25 January 2019An ability that triggers when counters are put on a permanent will trigger if that permanent somehow enters the battlefield with those counters.
- 25 January 2019If a creature somehow loses all of its +1/+1 counters, it can adapt again and get more +1/+1 counters.