Fair warning, I don't suggest doing this because you'll end up way over powered and steamroll through a section of the game, but nevertheless there is a pretty quick method to boost your Pokebucks and your Pokelevels for the midgame and all you need is full access to Lumiose City and $15,000 Pokebucks.
In Lumiose City there are restaurants that serve as battle places because it's Pokemon and you can't enjoy a nice French meal without having to beat the wait staff and the chefs. The restaurant you want is the two star, Le Yeah which can be found by going down the first street when taking a left out of the North Avenue Pokecenter.
Le Yeah has two options, triple battle and rotation battle. Pick triple.
Now comes a series of four battles with each opponent having Pokemon at level 31.
Battle 1: Sigilyph, Emolga, Mantine
Battle 2: Dugtrio, Pidgeot, Shuckle
Battle 3: Three Zangooses (Zangeese?)
Battle 4: Simipour, Simisear, Simisage
With every battle you're going to be getting about 1,100 exp per Pokemon you take out, more if your using a traded Pokemon. It might not seem like much, but it adds up really quickly and it only takes about three minutes, depending on if the A.I. uses Protect all the time. I went into one of them with a level 40 bunch and now I have them at level 60 after only an hour and a half or so. You're bound to have your whole party level up during the course of the battles. Possibly even multiple times.
After winning your battles you are given about 12 to 14 Big Mushrooms. You want these. Take these to a store and sell them. You'll get around 30k Pokebucks to add to your winnings from the four battles. It might not seem like much of a profit, but you can easily leave with 100k+ in pretty much no time at all. Using the Reward Money O-Power will also increase profit from the battles.
It might not be the ultimate fastest method, but it is a method to consider if you're having trouble with a gym or something, or if you just want to buy that 200,000 outfit at the Lumiose boutique. Try it some time. Or don't. It's up to you how you do your Pokemaning.