It's completely about the Lumiose style system... Apparently no one ever told Serena that she could get hefty discounts for spending time about the city attractions.
For clarification, the megastone the old man sells you is normally set at 1 million pokedollars, if you max out your style it goes down to 10k.
You normally gains style by doing all the thing mentioned by the three girls in the first panel and more, such as shopping in the specialty stores or buying a galette. Since shoping style is accumulated for each purchase, and not the total amount you spend. You can easily max out style by buying a lot of a cheap item, as shown by brown skin Serena and her cartfull of galettes
Hii--! Ten thousand yen would be alright!Let's hit the salon!Want to go check out the boutique?GalettesIt's all okay...'Cause I'm a Pokemon Trainer...T-there's something I wanted to buy so...SerenaI DID IT-! I finally bought it~!Gimme a stone.Wouldn't you like to go to the art museum?Excuse me, a Mega Stone please!Yes, one million yen.