Why do the Straw Hats never have money?
Mostly it's because Luffy always spent their money on food. Almost after every major battle Mugiwara Pirate always have a party and use almost every food they have. This will leave them with no food so they spent their money on food in the next town. For once they won a jackpot. They got 300mil belly. But it was used for their new ship, Thousands Sunny.

The other crew are not so loyal but they also spent a good sum of money. Nami on clothes, Robin and Chopper on book, Sanji on ingredients, and Ussop on miscellaneous stuff. (We know they did on Logue town and Water 7)