B.K.
Preliminary exams are far from useless. They will tell you EVERYTHING. And then you have the knowledge to make the correct decision. I would at least want to fix that cough if it's that bad, and they will have to know what it is in order to fix it. It very well could be cancer. And if so she could be suffering a great deal. If you can't afford the tests in order to find out what is wrong, then the best thing would be to euthanize her. Just in case she's suffering, end it somehow for her. I am not at all for heroics either if a dog is old and has cancer. They don't understand why they are suffering and I think it's better to just let them go. Hugs to you because this is so hard.