What is the highest amount of money you can lose in a single battle?

Question

How much money can you lose?

Answer 1

This how money lost is calculated when you black-out:

Where level is the level of the player's highest-level Pokémon.
The NPC with the highest base payout is Diantha (the Kalos Champion, Source) with 240.
So if your highest level Pokemon is level 100 and you lose to Diantha you would lose:
#100x240=24000

Source