Should I put my dog to sleep.?

I have a rott and he's eleven years old. He has cancer in his foot, it's the size of a tennis ball. We spun over 500 dollars on mess and treatments and it's just get worst. I dont have 5000 to do the cut the leg off operation that nt even a promise that he'll live. I would feel horrible to put him to sleep #TELL ME WHAT YOU THINK"! Oh but then again he very stable, only thing wrong is his front legs