I'm so sorry to hear about your dog.
Our dog didn't have cancer, but she had an enlarged heart. We were spending $300+ a month for vet visits and prescriptions to help keep her comfortable. She would have passed away a few years prior had it not been for the meds.
Towards the end, she could barely manage to walk and was losing control of her bladder/bowels. The night before we had her put down, we fed that dog everything you could imagine... twinkies, vanilla wafers, etc. She was curled up next to us the entire night, loving every bit of the attention. The next morning was extremely tough, but we knew that we were doing the right thing. I miss my little girl.