I'm gonna do math (not my strongest suit, but I'll try ;3)
The highest money should be defeating Diantha in the battle chateau with a Black Writ of Challenge, using a level 3 Prize Money O-Power and a 100 level Smeargle with Happy hour and Payday, holding an Amulet Coin.
So here's the math:
Original prize money: 19,200
Amulet Coin and Happy Hour (effectively quadrupling the original, as both double prize money):
19200 x 4 = 76,800
O-Power: 76800 x 3 = 230,400
Payday (the amount will increase every time you use it, so it depends, in this case, we take once):
230400 + 500 = 230,900 pokedollars
Note that your Smeargle has to be level 100 as Payday increases your prize money with this formula:
prize money increase = 5 x level
Source - My greymatter lol
Hope I helped!