How can I predict how much money I will lose if I lose a battle?

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.

Badges/Payout

• 0/8
• 1/16
• 2/24
• 3/36
• 4/48
• 5/60
• 6/80
• 7/100
• 8/120

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.

Source (bulbapedia)

While it is possible the bulbapedia article is outdated given the site's recent downtime, serebii's battle changes page lists no change to the payout system in gen V.