Right now I'm watching the 139th episode and I'm wondering why they are always in need of money. I don't remember exact cases when they got a jackpot, but I have a feeling that there were some cases; I just can't remember them. Do they really engage in such bad ass affairs but then remain out of money?
Examples that I remember where they showed a lack of funds:
Zoro in Loguetown buying swords
Luffy in Alabasta talking to old man with dance powder
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)
They always hit jackpots. I am not sure what arc you are up to, but they make lots of money on Alabasta, Thriller Bark, and so on. The problem is that they always have to spend it; buying the Sunny, etc. So they end up broke again. In fishman Island, they got lots of treasure but spent it all before they even left because something happened (You're not up to there so I won't say". The same process pretty much repeats every arc.