There are only three Mega Stones you can buy, and they are:
Venusaurite: 10,000-1,000,000 *(If Bulbasaur was not chosen)
Charizardite X/Y: 10,000-1,000,000 *(If Charmander was not chosen)
Blastoisite: 10,000-1,000,000 *(If Squirtle was not chosen)
The price depends on how much Lumiose Style you have. You can increase your style drastically by buying 99 Premier Balls from the Poke Mart inside the Poke Center.
You must find all of the other Mega Stones either by them being given to you or finding them with the Upgraded Mega Ring (more info here) between 8-9 PM. Locations told here.
As for money, going into the Battle Chateau, writing a Gold Writ of Invitation while making your lead Pokemon hold an Amulet coin gives good money over the course of the day when more trainers come. Battling this with Happy Hour Inkay or Pay Day Persian increases the value of this even more.
Battle Chateau + Amulet Coin + Gold Writ of Invitation = INSTANT CASH
~ Dec 10, 2013 by Celestial★Hurricane
You can also battle in Restaurant Le Wow in Lumiose perfectly (following the amount of turns) to get 25 Balm Mushrooms, which sell for 156,250. You can also scavenge for Balm Mushrooms in trashcans in the Pokemon Village.
I find Restaurant Le Wow faster in making money. Dem shrooms sell for about ~150k.
~ Jan 22, 2014 by fondant