Oh, and please don't get me wrong, Luna's health will always be my main concern, not the money, but I know that many here have been at this point in the past, so you will probably understand...
You are most certainly NOT an irresponsible pet owner!

I was at that point when I had Marina Mar. She was so sick. We did all kinds of tests. She was at the vet every other day for blood work. When they finally realized what it was, they told me that the last resort would be kitty chemo. I said absolutely not. I loved her with all my heart and could not put her through daily blood draws and her being even more sick than she already was. She was only 3 1/2.

Oh and for the record, I would NEVER say that to someone. That's just plain cruel.

You do what YOU feel is right. No one here can fault you for thinking about what's in your pets best interest.