weird bump on my dogs back

so my dog has had this bump on his back for a while, and it wasn't really that big before but we had his fur shaved for the summer and now that we can see it without his fur covering it and we see that's it's kinda huge, I want to take him to the vet but we took him to the vet once because he kept gagging and acting weird and they charged us like $600 just to tell us that he swallowed a rock and he'll be fine and we can't afford to do that again. anyone have any idea what it could be? he doesn't really react if you touch it