Yes, it works in wild battles as well.
>Pay Day does damage and scatters coins on the ground with a value equal to twice the user's level for each time it is used. These coins are picked up afterwards if the player wins the battle.
It was changed to 5x the user's level in generation 3. Amulet coin also doubles the amount of money gained.
If you level up during the battle, it applies the new level in this calculation instead of the old one.
Hope I helped.