My dog is VERY sick any chance she can live?

For the past 2 weeks my dog has been sick. for the 1st 3 days she was sick all she did was puke and not eat but then after that she started to move very slow and lose weight so we took her to the vet and they stuck something in her back like an I.V and it cost 150 bucks! This did nothing but make my dog in more pain and she screamed and 3 more days later she puked again so we took her back up and this time they took her blood to do test to see if she has cancer or anything else. That cost another 200 bucks! Now they are saying that she might have been poisoned and that it is going to cost 500 bucks to do surgery or she will die. :( i cried when i heard that from the vets mouth. . . . She now coughs , gags and she just drinks water (i know this is good that she drinks water) and she wont even eat her jucie fave dog food :( i dont know what to do! My dog is my family! I can now see almost all of her bones expesially her back bone. What can i do? Should i wait a few more weeks days months? The vet wants me to uthanized her!! What im going to do is hit this vet in her face the nextime i see her! I would go to a nother vet but i calld them and they would just do the same thing this vet did! :( please help!