Cheapest priced copy (Wilds of Eldraine Promos): $0.49 at Card Kingdom, near mint, their price list as of .
Raging Battle Mouse 
Creature — Mouse · 2/1 · Rare · Wilds of Eldraine · #143
The second spell you cast each turn costs {1} less to cast.
Celebration — At the beginning of combat on your turn, if two or more nonland permanents entered the battlefield under your control this turn, target creature you control gets +1/+1 until end of turn.
Legal inCommanderBrawlStandardPioneerModernLegacyVintage
Not legal inPauper
4 printings · $0.49 to $0.79 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Wilds of Eldraine Promos | 143p | Rare | $0.49 | $0.49 |
| Wilds of Eldraine Promos | 143s | Rare | — | $0.69 |
| Wilds of Eldraine | 143 | Rare | $0.49 | $0.49 |
| Wilds of Eldraine | 349 | Rare | $0.79 | $0.49 |
9 rulings from Wizards of the Coast
- 1 September 2023The permanents that entered the battlefield don't need to remain on the battlefield or under your control. Celebration abilities are checking for past events, not the current game state.
- 1 September 2023Each celebration ability has a different effect.
- 1 September 2023Raging Battle Mouse can't reduce its own cost, even if it's the second spell you cast in a turn.
- 1 September 2023Raging Battle Mouse's cost-reduction ability doesn't change the mana cost or mana value of any spell. It changes only the total cost you pay.
- 1 September 2023Spells that were cast before Raging Battle Mouse entered the battlefield count. If Raging Battle Mouse was the first spell you cast this turn, the next spell you cast this turn is your second spell.
- 1 September 2023Celebration abilities only care if two or more nonland permanents entered the battlefield under your control in a turn. They won't get more powerful if more than two permanents entered the battlefield under your control in a turn.
- 1 September 2023Some celebration abilities trigger at specific parts of the turn and check whether two or more nonland permanents entered the battlefield under your control already in that turn.
- 1 September 2023Others are static abilities that give creatures abilities or power and toughness increases as long as two or more nonland permanents entered the battlefield under your control that turn.
- 1 September 2023Raging Battle Mouse's cost-reduction ability can't reduce the amount of colored mana you pay for a spell. It reduces only the generic mana component of that cost.