where do you think phenieas and ferb get all the money to buy everything that they make???????

But most of the time they have to order all the supplies, and when they sign, the delivery guy says "Aren't you a little young to be *blank*?" And Phineas says "Yes. Yes I am."

They're probably more responsible with their money, and have been saving up sice birth for this one super amazing never-ending summer. I don't know. It depends on what they're building. They're also pretty resourceful with what they have, so they don't need to be spending too much. It's also a TV show, so maybe they're money comes from thin air.