Cheapest priced copy (Battle for Zendikar): $6.49 at Card Kingdom, near mint, their price list as of .
Conduit of Ruin 
Creature — Eldrazi · 5/5 · Rare · Battle for Zendikar · #4
When you cast this spell, you may search your library for a colorless creature card with mana value 7 or greater, reveal it, then shuffle and put that card on top.
The first creature spell you cast each turn costs {2} less to cast.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
2 printings · $6.49 to $18.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Battle for Zendikar | 4 | Rare | $6.49 | $13.99 |
| Battle for Zendikar Promos | 4s | Rare | — | $18.99 |
9 rulings from Wizards of the Coast
- 25 August 2015Conduit of Ruin's last ability doesn't change the mana cost or mana value of any spell. It changes only the total cost you actually pay.
- 25 August 2015The first creature spell you cast each turn doesn't necessarily have to be the first spell you cast. You could cast a sorcery spell and then cast a creature spell that would get the discount.
- 25 August 2015Conduit of Ruin's last ability will look at the entire turn, even if Conduit of Ruin wasn't on the battlefield for some of it. Notably, if you cast Conduit of Ruin in a turn, then no other creature spell you cast that turn can be your first.
- 25 August 2015The cost reduction can apply to alternative costs (such as dash costs).
- 25 August 2015If the first creature spell you cast in a turn has {X} in its mana cost, you choose the value of X before calculating the spell's total cost. For example, if the first creature spell you cast in a turn has a mana cost of {X}{G}, you could choose 2 as the value of X and pay {G} to cast the spell.
- 25 August 2015If you control more than one Conduit of Ruin, the last ability of each of them applies only to the first creature spell you cast each turn, not to different spells; the first creature spell you cast each turn will cost {4} less.
- 25 August 2015If there are additional costs to cast a spell, or if the cost to cast a spell is increased by an effect, apply those increases before applying cost reductions.
- 25 August 2015Conduit of Ruin's last ability can't reduce the amount of colored mana you pay for a spell. It reduces only the generic component of that mana cost.
- 25 August 2015If the first creature spell you cast in a turn has converge, you can't ignore the cost reduction of Conduit of Ruin's last ability in order to spend more colors of mana.