Need advice about putting cat down...?

Yesterday I found out that my very miserable and sick 12 year old cat has kidney stones, bladder stones, possible kidney failure, possible diabetes. I spent over $1,000 already and haven't even done much other than tests and fluids. I don't want to put her through a bunch of surgeries and crap just to prolong her life and make her suffer. I also didn't have the $1,000 and don't know how much more into debt I can put myself. I feel horrible but I'm on a very tight budget. More importantly though, I don't want her to suffer. What would you do if you were me?