I have an 18 yr old mini pincher. She's been breathing heavy for awhile, but since she was eating & drinking water with no problem didn't think anything of it. Today she hasn't eaten and only drinks water, the vet wants me to spend $500 to find out what's the problem. Would u spend the $?

That is totally your choice, at 18 she has had a really long life.
But if this were my dog and she was doing well otherwise and the breathing seemed to be her major issue. I would spend the money to see if there were non invasive things that could be done to make her remaining days comfortable, medications etc.
On the other hand if it is something is found that cannot be treated and will only get worse in the near future, then you have that knowledge on board to help make decisions for her that will allow you to ease any suffering that she may endure, should it come to that.
Sorry to bring that up but with an 18 year old pet it is something to keep in mind.
Personally, I prefer to have an answer and work with it rather than leaving things to chance.
Good luck, I wish you two the best

I would. I agree 18 years is a good long life, and there might not be much a vet can do even after spending the money. I would also spend the money to have peace of mind knowing she isn't suffering, so if after I spent the $500 to find out my dog had lung cancer or some other terminal illness I would shell out what ever else it would cost to have her put down to ensure as little suffering as possible. The way I see it is your dog gave you 18 years of her best, she needs you to take care of her in her old age. It's never an easy decision, and I hope you can find out what's wrong with as little cost to you as possible. Are there other vets you can call to price check with?

I'd spend the money... Or at least pick. Unless your vet is majorly expensive, 3 radiographs and a blood test wouldn't cost $500. Those items may cost $250-$300.