Cheapest priced copy (The Lost Caverns of Ixalan Commander): $0.99 at Card Kingdom, near mint, their price list as of .
Simic Ascendancy 
Enchantment · Rare · Ravnica Allegiance · #207
{1}{G}{U}: Put a +1/+1 counter on target creature you control.
Whenever one or more +1/+1 counters are put on a creature you control, put that many growth counters on this enchantment.
At the beginning of your upkeep, if this enchantment has twenty or more growth counters on it, you win the game.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
6 printings · $0.99 to $13.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Bloomburrow Commander | 260 | Rare | $1.29 | — |
| The Lost Caverns of Ixalan Commander | 288 | Rare | $0.99 | — |
| Ravnica Allegiance Promos | 207 | Rare | — | $2.29 |
| Ravnica Allegiance Promos | 207s | Rare | — | $4.99 |
| Ravnica Allegiance | 207 | Rare | $0.99 | $4.99 |
| Secret Lair Drop | 1832 | Rare | — | $13.99 |
4 rulings from Wizards of the Coast
- 25 January 2019If Simic Ascendancy doesn't have twenty or more growth counters on it as your upkeep begins, its last ability won't trigger. You can't take any actions during your turn before your upkeep begins.
- 25 January 2019If the last ability does trigger, but Simic Ascendancy leaves the battlefield, use the number of counters it had on it immediately before it left the battlefield to determine whether you win the game.
- 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 the last ability does trigger, but counters are removed from Simic Ascendancy so it has fewer than twenty remaining on it, you won't win the game.