Cheapest priced copy (Modern Masters 2017): $0.79 at Card Kingdom, near mint, their price list as of .
Summoning Trap 
Instant — Trap · Rare · Modern Masters 2017 · #139
If a creature spell you cast this turn was countered by a spell or ability an opponent controlled, you may pay {0} rather than pay this spell's mana cost.
Look at the top seven cards of your library. You may put a creature card from among them onto the battlefield. Put the rest on the bottom of your library in any order.
Legal inCommanderModernLegacyVintage
Not legal inBrawlStandardPioneerPauper
2 printings · $0.79 to $6.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Modern Masters 2017 | 139 | Rare | $0.79 | $1.99 |
| Zendikar | 184 | Rare | $0.99 | $6.99 |
4 rulings from Wizards of the Coast
- 1 October 2009Casting a Trap by paying its alternative cost doesn't change its mana cost or mana value. The only difference is the cost you actually pay.
- 1 October 2009Effects that increase or reduce the cost to cast a Trap will apply to whichever cost you chose to pay.
- 1 October 2009You may ignore a Trap's alternative cost condition and simply cast it for its normal mana cost. This is true even if its alternative cost condition has been met.
- 1 October 2009If there are seven or fewer cards in your library, you'll look at all of them.