Cheapest priced copy (Commander Masters): $0.39 at Card Kingdom, near mint, their price list as of .
Portal Mage 
Creature — Human Wizard · 2/2 · Uncommon · Commander Masters · #112
Flash
When this creature enters during the declare attackers step, you may reselect which player or permanent target attacking creature is attacking. (It can't attack its controller or their permanents.)
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
3 printings · $0.39 to $0.79 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Commander 2017 | 13 | Rare | $0.49 | — |
| Commander 2020 | 122 | Rare | $0.49 | — |
| Commander Masters | 112 | Uncommon | $0.39 | $0.79 |
5 rulings from Wizards of the Coast
- 25 August 2017If an ability targets something controlled by the “defending player” of an attacking creature and the defending player for that creature changes before that ability resolves, the ability won’t resolve because its target has become illegal.
- 25 August 2017You may cast Portal Mage outside of a declare attackers step. If Portal Mage enters the battlefield outside of a declare attackers step, its ability simply doesn’t trigger.
- 25 August 2017Reselecting which player or planeswalker a creature is attacking ignores all requirements, restrictions, and costs associated with attacking.
- 25 August 2017Reselecting which player or planeswalker a creature is attacking doesn’t cause “whenever this creature attacks” abilities to trigger. Notably, the Curses in this set (such as Curse of Vitality) won’t trigger while Portal Mage’s ability is resolving.
- 25 August 2017If you reselect which player or planeswalker an attacking creature is attacking, that creature is still considered to have attacked the player or planeswalker as declared, but it is now attacking the new player or planeswalker.