We don't really know how poor the Saotomes are.
Thievery.
They might not be that poor since they do own a house in Tokyo which was not their original house (Ranma's mom tells Ryu that the house they lived in was torn down)
Compensation by the government for taking the house for a project.
travel a lot (Ranma's mom does as well since we see Ranma and Genma bump into her on a trip)
Stowing away on trains/trucks/etc.
she had no trouble buying outfits for 'Ranko'
Nodoka might have spare funds, Ranma probably 'borrows' from school club or something.
eating out
You can actually eat out on the cheap in Japan and still get good food if you're careful.
going to the spa
IIRC, that was a family trip with the Tendo's so it would probably be a rare vacation in real life. The other trips to hot springs were 'jobs'.
frankly Ranma wears expensive clothing often(|kimonos, tuxedo's, really expensive looking dresses, etc)
Without a specific instance to reference against, I'd have to put money on them being rented or stolen by Genma depending on context if it was for the 'marrage' and being borrowed/stolen for one of Ranma's quirky plans if female clothing.
Most of the evidence points to the Soatome's or at least Ranma's mom not being poor in general.
Inconclusive.
This could be explained in a number of ways but I'm not going to really try to do so though the easiest explanation for the incongruities is that Genma is terrible with money whereas Ranma's mom is good with it and tries to save as much as she can
Inconclusive.
Ranma tells Genma to change back from being a panda a lot more than Genma tells Ranma.
True, more for the fact that Ranma doesn't want to bother reading the signs or it's a gag joke where someone cant 'hear' Genma talk in bear form.