I never ran out of money in any of my other games (all of them Gen 4 and 5) but I seem to run out money all the time in X. Is it there's just more cool stuff to buy and I have been throwing money at all of it, or do you get less prize money comparatively?
Actually, you gain less prize money after battles than you did in other Pokemon games. I suggest using an Amulet Coin + Prize Money O-Power (preferably Level 3, but any Level will do) to gain lots of money. Here is a thread from Marriland which may be helpful to you.
Source: My knowledge of playing Pokemon Blue, Gold, White, and Y. (Never played generation 3 and 4 :P)