Spend the money. You'll know as you start writing the check whether it is the correct thing to do. You may or may not save the cat, but if you can't let him go, then you can't let him go.
I spent close to $800 to save my precious 11 year old cat. She has given me so much love, there is nothing I wouldn't do to help her. She's 11 and pulled through with flying colors!