There has been a lot of speculation on this issue. There really isn't an
answer, Takahashi probably never really considered this, it isn't important
to the plot.

I agree with this. She wrote the series (at this point) for comedy. Real
life impact wasn't considered since Ranma 1/2 is supposed to be an escape
from reality.

After all, they can jump tall buildings in a single bound... what's a
couple of bucks to fix the place up. These aspects were simply not
important to the story she was trying to tell, so they were simply ignored.

that) so there is some money coming in that way. There may be classes going
on that just aren't shown. Soun mentions (in "like water for Ranma") that
he has no regular source of income, so they probably just get money from
lots of little things and Nabiki and Kasumi probably are masters at
stretching a buck. 

The part about Kasumi knowing how to stretch a buck is very believable. 
About Nabiki... no way.

<FANFIC ANTI-CLICHE RANT>
Nabiki has no altruistic reasons for doing the things she does. In the
aforementioned episode "Like Water for Ranma", her solution to the problem
was to order out the most expensive thing possible. When Soun complained,
she turned her nose up at him and walked off. Her solutions are always
expensive... wether you pay her directly or not.

Many fanfics showcase Nabiki as the Scrooge with a heart of gold who hoards
all that money to take care of the house and family. The fact that she
nearly financially destroyed her own family for the sake of challenge (in
the manga so this is canon) indicates that she only looks out for herself.

My point here is that most new readers are taking the fanfic representation
of her as canon... and it is not. She's as cold as they come
</FANFIC ANTI-CLICHE RANT>

Now... I have nothing against casting her in a different light (it's YOUR
story after all), but things like Akane grabbing a mallet out of nowhere at
the slightest provocation and Shampoo always poisoning Ranma are pretty
much fanfic cliches and are not canon.

  ___________
 / Here lies:\  First Corollary of Taber's Second Law:
|   Andrew    |    Machines that piss people off get murdered.
|             | 
|   I think   | 
| he's hungry | The shortest distance between two points is 
|             | under construction.            -- Noelie Alito