If I lose a battle (i.e. all 6 of my pokemon faint), how much money will I lose?
The amount of money you will lose since FireRed and LeafGreen (with the exception of Emerald) is determined by:
Money = Level * Base Payout
Level is the highest level pokemon of yours and the base payout is based on the number of badges you have.
For example, if you have 5 gym badges and your top level pokemon is 30, your money lost for defeat would be 30 * 60 = 1800.
While it is possible the bulbapedia article is outdated given the site's recent downtime, serebii's battle changes page lists no change to the payout system in gen V.