Welcome!
When my Cinderella got sick, I spent my last dollars trying to diagnose, treat and save her (I wasn't working and had no income). It ended up that she was terminal, but I don't regret spending the money. I would have always regretting not trying. But it's a decision everyone has to make themselves.
One calico, orange and white color who was 7 years old died a week ago from a tumor cancer.
You really don't know that, though, since you didn't run the tests.
So, I was wondering what would you do if you were in our shoes.
I certainly wouldn't take in another cat, especially a stray, who could not only have something wrong with her, but could also expose your other cats to illness and/or diseases, when your finances are limited for vet care.
I was wondering, though, would you have spent the money for the vet if it was one of your persian cats who got sick?