Lost dog in need of medical

We had this beautiful girl show up on our door step. She is sweet and gentle with puppies and cats so we decided to take care of her till we can find her owner. We started to notice that where she laid that there was blood where she would sit. So we called the vet to find out how much it would cost and with tests and everything would be around 400 dollars to make sure there is nothing seriously wrong with her. It could be nothing serious but I would rather be safe then sorry and if she is in any kind of pain and just not showing it would rather her be better.  If you can donate anything that would be  great or if you can share would be great as well.  Thanks so much for your help with this fur baby.