Does who have the cancer hemangiosarcoma do not experience any pain.
The hemangiosarcoma cancer is an almost exclusive cancer to dogs and it causes no pain for the dog experiencing the hemangiosarcoma cancer.
If the dog does not get treatment for the hemangiosarcoma then the dog could die within 1 to 4 weeks after diagnoses depending on how far progressed the hemangiosarcoma has become in the dog.
Although it's a terrible thing for the dog to have hemangiosarcoma it's not a painful way for the dog to die.
But if your dog does have hemangiosarcoma and you cannot afford treatment for the hemangiosarcoma in the dog then you can either let the dog die slowly or have the dog put to sleep.
I would opt to put the dog to sleep if I could not afford the treatment even though the dog is not in pain.