What is the maximum amount of Poke dollars you can hold?

I don't know exactly what the limit is, but my wallet has gone above the 1 million mark, so it is definitely possible to purchase them with sufficient grinding. Also relevant, I came back to the stone salesman after beating the league once, and his price was only 500,000. After purchasing the stone from him, he'll stock the third kanto-starter mega stone.

The price cut might not be related to the league victory, I didn't really check back there between the 4th gym and the end.

The new limit is 9,999,999 pokedollars, which i found out by having too much free time and over using the prize money o-power. The mega stone prices go down because of how famous you are in lumiose i believe, which is increased by your first league victory...i have seen prices for them go down to 150,000...either this is lumiose fame or they used a lvl 3 o-power, im not certain.