Dr. Zoidberg is known to be very poor. He lives in a dumpster and eats garbage. But wouldn't he be paid similar amount to say, Fry, Bender, Leela, and Scruffy?
Fry, Bender, Leela, and Scruffy all seem to do okay, albeit on the poorer side of the spectrum, but are still able to pay the rent on their apartments and feed themselves. Shouldn't Zoidberg be able to do the same?
The practical answer is that he's poor for comedic purposes.
Zoidberg's characteristics juxtapose him being a doctor while being poor, and him having strong Jewish characteristics while being a shellfish and having poor financial management skills (a Jewish stereotype).
Because he's incredibly bad with financial decisions.
In the episode where Planet Express's stocks are being inflated by a 1980's businessman, he sells his stock in the company for a sandwich, fails to refrigerate it, and then eats it.
And in the series finale (the first time), he buys eight copies of a two-record set for no apparent reason, as well as eight copies of a newspaper featuring the opera he had just witnesses himself.
This, combined with him essentially being made to pay off outrageous sums by his friends and not appearing to be paid well at all by the Professor (along with the other members of the crew) indicates that he's not only poor, but the money he does have he spends very unwisely.
The Futurama wikia mentions several reasons:
Well, to answer you question, dr. Zoidberg has no alternative funds (e.g. Bender steals) and is probably the most useless member of Planet Express crew. Also, in episode "Future Stock", it's shown that he spent all of his shares of the company. Therefore, it's perfectly logical to assume that he's the poorest one. And another thing, Zoidberg is Professors oldest friend, and their relationship is less work-related than that of others.
The Futurama wikia mentions several reasons:
He is extremely poor and has stated several times that he has no home. Hermes Conrad considers him as practically a slave. Everyone makes him pay for stuff (even though he has no money and people will not pay him any). A conversation Zoidberg has with the Professor suggests that he is only staying with Planet Express because he is a slave, and must "earn his freedom." (References to The Beast with a Billion Backs) When Zoidberg recieved $300 from the mass tax refund, he thought he was rich.
To add on what Izkata, Zoidberg has no value for money. When Fry was a Holophoner musician, Zoidberg bought extra items on two occasions: Newspaper, and a 2-record set. Zoidberg also has bad credit(his credit card application was denied).
In another occasion, Fry and Bender flew the Planet Express Ship causing the entire Planet Express building to follow the ship cause major damage. Hermes then said that Zoidberg had to pay for the damages.
All in all, Zoidberg is the Millhouse of Futurama. He always gets picked on, and he's the type of character that wont improve in his situation.
Back then we elaborated a reason.. is that he expend all his money when he was going to die. the thing is that the character starts with aparently the same economic status as everyone else in the series.. he is seeing paying money with no trouble and going to restaurants and hotels .. up until the episode where he goes to his home planet to "mate and die" ..in the end he doesnt mate and doesnt die.. and from that episode on the "Zoidberg is poor" thing began..
Obviusly is a case of "Flanderization" as the writers found it funny to make Zoid poor and began to feast on that.. but we found funny the coincidence that he was suposed to die (and he knew it well) and from there he is poor.. and especulated he knew he was going to die so he just expended all his money
