Cheapest priced copy (Dominaria United Commander): $1.79 at Card Kingdom, near mint, their price list as of .
Urza's Ruinous Blast 
Legendary Sorcery · Rare · Commander Masters · #842
(You may cast a legendary sorcery only if you control a legendary creature or planeswalker.)
Exile all nonland permanents that aren't legendary.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
6 printings · $1.79 to $10.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| The Brothers' War Commander | 79 | Rare | $1.99 | — |
| Commander Masters | 842 | Rare | $1.99 | — |
| Dominaria United Commander | 107 | Rare | $1.79 | — |
| Dominaria | 39 | Rare | $2.29 | $8.99 |
| Dominaria Promos | 39s | Rare | — | $10.99Out of stock |
| The List | DMC-107 | Rare | $1.99 | — |
2 rulings from Wizards of the Coast
- 27 April 2018You can't cast a legendary sorcery unless you control a legendary creature or a legendary planeswalker. Once you begin to cast a legendary sorcery, losing control of your legendary creatures and planeswalkers won't affect that spell.
- 27 April 2018Other than the casting restriction, the legendary supertype on a sorcery carries no additional rules. You may cast any number of legendary sorceries in a turn, and your deck may contain any number of legendary cards (but no more than four of any with the same name).