Cheapest priced copy (The Lost Caverns of Ixalan Commander): $0.49 at Card Kingdom, near mint, their price list as of .
Benthic Biomancer 
Creature — Merfolk Wizard Mutant · 1/1 · Rare · Ravnica Allegiance Promos · #32p
{1}{U}: Adapt 1. (If this creature has no +1/+1 counters on it, put a +1/+1 counter on it.)
Whenever one or more +1/+1 counters are put on this creature, draw a card, then discard a card.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
4 printings · $0.49 to $6.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| The Lost Caverns of Ixalan Commander | 145 | Rare | $0.49 | — |
| Ravnica Allegiance Promos | 32p | Rare | $0.79 | $6.99 |
| Ravnica Allegiance Promos | 32s | Rare | — | $3.99Out of stock |
| Ravnica Allegiance | 32 | Rare | $0.49 | $2.49 |
4 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 2019You draw a card and discard a card all while Benthic Biomancer's triggered ability is resolving. Nothing can happen between the two, and no player may choose to take actions.
- 25 January 2019If a creature somehow loses all of its +1/+1 counters, it can adapt again and get more +1/+1 counters.