Cheapest priced copy (Commander Masters): $0.99 at Card Kingdom, near mint, their price list as of .
Oath of Gideon 
Legendary Enchantment · Rare · Oath of the Gatewatch · #30
When Oath of Gideon enters, create two 1/1 white Kor Ally creature tokens.
Each planeswalker you control enters with an additional loyalty counter on it.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
3 printings · $0.99 to $4.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Commander Masters | 830 | Rare | $0.99 | — |
| Oath of the Gatewatch | 30 | Rare | $1.49 | $2.99 |
| Oath of the Gatewatch Promos | 30s | Rare | — | $4.49Out of stock |
3 rulings from Wizards of the Coast
- 22 January 2016Each of the Oaths is legendary: if you control more than one of any particular Oath, you choose one to remain on the battlefield and the others are put into your graveyard.
- 22 January 2016The “legend rule” checks the full names of legendary permanents. You can control Oath of Gideon and Oath of Chandra at the same time, for example.
- 22 January 2016Even if you control a particular Oath, you can cast another of that Oath to take advantage of its enters-the-battlefield ability. It will enter the battlefield, causing its first ability to trigger. You will then choose one to keep, as detailed above, and then its enters-the-battlefield ability will resolve.