Armies eat through money like you wouldn't believe and before the rise of modern banking institutions, it was extremely difficult to raise funds for a protracted campaign. I don't know if Arab armies in this time differed much from Europeans in how they raised funds, but I can say that European rulers in the time of the Crusades had to go to extreme measures to get the necessary cash. This usually involved selling or loaning out their land to monasteries for a set number of years, selling titles, taking loans from Jewish moneylenders (or outright stealing it) and gathering whatever they could through new taxes. Even then, most Crusaders who came back alive tended to be near penniless.