Cheapest priced copy (The Lost Caverns of Ixalan Commander): $4.49 at Card Kingdom, near mint, their price list as of .
Runic Armasaur 
Creature — Dinosaur · 2/5 · Rare · The Lost Caverns of Ixalan Commander · #256
Whenever an opponent activates an ability of a creature or land that isn't a mana ability, you may draw a card.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
4 printings · $4.49 to $49.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| The Lost Caverns of Ixalan Commander | 256 | Rare | $4.49 | — |
| Core Set 2019 | 200 | Rare | $5.49 | $9.99Out of stock |
| Core Set 2019 Promos | 200p | Rare | $12.99 | $49.99 |
| Core Set 2019 Promos | 200s | Rare | — | $8.99Out of stock |
4 rulings from Wizards of the Coast
- 13 July 2018Runic Armasaur's ability doesn't trigger when an opponent activates an ability of a card in hand (such as a cycling ability from the Amonkhet block) or a card in a graveyard (such as that of Bone Dragon), even if that causes a card to be put onto the battlefield.
- 13 July 2018Activated abilities contain a colon. They're generally written "[Cost]: [Effect]." Some keyword abilities are activated abilities and will have colons in their reminder text. An activated mana ability is one that produces mana as it resolves, not one that costs mana to activate.
- 13 July 2018Runic Armasaur's ability resolves before the ability that caused it to trigger. Players can cast spells and activate abilities after the triggered ability resolves but before the activated ability that caused it to trigger does.
- 13 July 2018Runic Armasaur's ability doesn't trigger when an opponent activates an ability of a noncreature, nonland permanent that causes it to become a creature (such as a crew ability of a Vehicle).