Every person you battle gives you a fixed amount of money. There is no calculation. Like what Cobragon said, the more authority they have, the more money you'll receive. This includes Rich (insert gender here)s. If by any chance you have Black and White, there is a rich family in Undella Town that will ask to battle you everyday - they'll give a lot of money.
As for your second question, no, multiple Amulet Coins and Luck Incenses do not mean an increased prize.
No, as far as I can tell, multiple Amulet Coins and Luck Incenses do not stack.
(Many other sources state that they don't stack, however I can't be bothered linking to them)
EDIT: According to Cobragon's source, apparently there is a formula, which was weird 'cos I could have sworn it was a fixed amount of money. This is, however, because it's almost always the same.
The formula is:
Payout = Level x Base
Base is your defeated opponent's title. For example, Rich Boy, PKMN Trainer, Champion, Janitor etc. Level is the level of the last (not strongest) Pokemon that was battled in the battle.
For example, if you were versing Champion Cynthia in Platinum, and she finished the battle with a Level 62 Garchomp, it would be $200 for Base and $62 for Level. The resulting prize money would be $12,400. :D Hopefully, that makes sense. Check Cobragon's source for what all the titles are worth. :3
Hope I helped. :)