Cheapest priced copy (Commander 2017 Oversized): $23.99 at Card Kingdom, foil, near mint, their price list as of .
The Ur-Dragon 
Legendary Creature — Dragon Avatar · 10/10 · Mythic · Commander Masters · #361
Eminence — As long as The Ur-Dragon is in the command zone or on the battlefield, other Dragon spells you cast cost {1} less to cast.
Flying
Whenever one or more Dragons you control attack, draw that many cards, then you may put a permanent card from your hand onto the battlefield.
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
8 printings · $23.99 to $229.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Commander 2017 | 48 | Mythic | — | $49.99 |
| Commander Masters | 361 | Mythic | $47.99 | $54.99 |
| Commander Masters | 594 | Mythic | — | — |
| Commander Masters | 689 | Mythic | $59.99 | $84.99 |
| Commander Masters | 1065 | Mythic | — | $229.99 |
| Commander 2017 Oversized | 48 | Mythic | — | $23.99 |
| MagicFest 2025 | 15 | Mythic | — | $39.99 |
| Secret Lair Drop | 11 | Mythic | — | $74.99 |
4 rulings from Wizards of the Coast
- 25 August 2017You may only put one permanent card from your hand onto the battlefield, no matter how many Dragons you attacked with or how many players they attacked.
- 25 August 2017You draw one card for each Dragon you controlled that attacked, even if some of them left the battlefield before The Ur-Dragon’s triggered ability resolves.
- 25 August 2017The Ur-Dragon’s triggered ability resolves after all attackers have been chosen. You can’t attack with some Dragons, put a creature card with haste onto the battlefield, and then attack with that creature. Similarly, any “whenever a creature attacks” abilities of the permanent card you put onto the battlefield won’t trigger.
- 25 August 2017If a Dragon you control enters the battlefield attacking, it won’t cause The Ur-Dragon’s last ability to trigger.