Right now in Pokémon Platinum I have around 10,000 or a bit less. I have not been playing the game as much I would wish to, so that's the cause for having such little money.
Seeing as I have been playing Pokémon Pearl a lot, I have around 650,000 or a bit more. Since I need money to buy Max Repels for chaining, I have been battling people a lot and that's how I gain most of my money. At other times, I sell a lot of the items I get from the Underground as I get one too many items of the same thing. Sometimes I sell items I don't use at all such as the X Attack or X Defend as well. Even though you don't have a limited amount of space in your bag, I like to make sure I don't have too much things to where I get lost on when I need to find what I need. On those cases, I usually sell uneeded things, or things I have multiple things of.