Cheapest priced copy (Revised Edition): $0.35 at Card Kingdom, near mint, their price list as of .
Fireball 
Sorcery · Common · 30th Anniversary Edition · #442
This spell costs {1} more to cast for each target beyond the first.
Fireball deals X damage divided evenly, rounded down, among any number of targets.
Legal inCommanderModernLegacyVintagePauper
Not legal inBrawlStandardPioneer
42 printings · $0.35 to $159.99 at Card Kingdom
4 rulings from Wizards of the Coast
- 17 November 2017Fireball's damage is divided as Fireball resolves, not as it's cast, because there are no choices involved. The division involves only targets that are still legal as Fireball resolves.
- 17 November 2017For example, if X is 5 and you choose three target creatures, Fireball has a total cost of {7}{R} (even though its mana cost is just {5}{R} and its mana value is 6). If those creatures are all still legal targets as Fireball resolves, it deals 1 damage to each of them.
- 17 November 2017You may cast Fireball with zero targets, regardless of the value chosen for X. If you do so, it will not be a targeted spell, and no damage will actually be dealt when it resolves.
- 17 November 2017You can target more than X creatures. However, if the number of legal targets at the time Fireball resolves is greater than X, none of them will be dealt any damage.