Just curious...
In the games, a Trainer pays out money based on a formula: Where Base is the base payout of the Trainer class, and Level is the level of the last Pokémon in the Trainer's party.
Note that the last Pokémon is not always the highest level Pokémon - for example, in Generation II, Red pays out 7700 even though there is a level 81 Pikachu in his party, because the last Pokémon he has is his level 77 Blastoise, and in Generation IV, he pays out 16,800 despite the level 88 Pikachu because, again, the last Pokémon in his party is his Level 84 Blastoise.
In Multi Battles, the total payout is simply the sum of the payouts of the two Trainers if they had separate parties.
Example: Fighting a Beauty in Gen 1 with a lv20 Pokemon as their last one will get you 1400 Pokedollars.
tl;dr: The level of the trainer's last Pokemon x Base payout for their class in the generation you are battling in
Source, check the chart for base payouts for each trainer class.