Older Cat Health Declining

I have a cat that is about 15 years old. She has had a history of urinary tract problems and has been treated with antibiotics for past utis. Now for the past week, it seems like her appetite is decreasing, she is drinking more as well as vomitting yellow bile and urinating outside the litter box. She just doesn't seem to be herself, sleeping more than usual, not getting up for snacks and such. Is this the beginning of the end, I don't want her to suffer and my family is on a strict budget and won't be able to take on costly vet visits, test and treatments.

I'm sorry, but it sounds like your cat is finally wearing out, from both age & being sick. 15 is a really good run for a cat - she's officially a senior cat now.
   
She is likely far sicker than you realize. You should take her to the vet, just to find out what the situation is.
   
I think any treatment would likely be very expensive & probably not successful, considering her symptoms. The vomiting is a very bad thing.
   
It may be time to let her go;
   
When you go to the vet, talk to him about paying any final bill over several months; these days, it's easier to get vets to agree to time payments.