Should we euthanize our cat?

We adopted him from a shelter 5 years ago and he is about 8 or 9 now. 4 years ago, he was diagnosed with degenerative disc disease and a ruptured disc. This has left him with a moderate limp and crooked spine. We treat with pain killers when it seems to bother him, but the only long term treatment is a $5000 surgery with a 60% chance of success. Last year, he started having seizures and our vet has been unable to find the cause. Since the onset of the seizures, he has become skittish and less energetic. Some days he cries all day and others he seems relatively fine. He rarely plays any more. My husband and I are really torn about this and I'm not sure that we can be objective about it, so I want to see what others have to say. TIA