Cheapest priced copy (The Lost Caverns of Ixalan Commander): $2.99 at Card Kingdom, near mint, their price list as of .
Descendants' Path 
Enchantment · Rare · Avacyn Restored · #173
At the beginning of your upkeep, reveal the top card of your library. If it's a creature card that shares a creature type with a creature you control, you may cast it without paying its mana cost. If you don't cast it, put it on the bottom of your library.
Legal inCommanderModernLegacyVintage
Not legal inBrawlStandardPioneerPauper
4 printings · $2.99 to $44.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Avacyn Restored | 173 | Rare | $4.49 | $15.99 |
| The Lost Caverns of Ixalan Commander | 238 | Rare | $2.99 | — |
| Secret Lair Drop | 1183 | Rare | — | $44.99 |
| Avatar: The Last Airbender Eternal | 167 | Rare | $2.99 | $2.99 |
3 rulings from Wizards of the Coast
- 1 May 2012If the revealed card is a creature card that shares a creature type with a creature you control, but you choose not to cast it, it's put on the bottom of your library. (This is a recent change.)
- 1 May 2012If you cast a card "without paying its mana cost," you can't pay any alternative costs. You can pay additional costs, such as kicker costs. If the card has mandatory additional costs, you must pay those.
- 1 May 2012If the card has {X} in its mana cost, you must choose 0 as its value.