I've not read all the replies. But my personal thoughts are - you didn't get horse vetted, you can't prove the dealer had any prior knowledge of the issue (would have flagged up had you had a vetting) and also if you send the horse back, it could be sold on.
I'm afraid I'd be PTS and cutting...
I'd feel comfortable offering as low as 70% of the horses advertised price under usual circumstances.
Definitely add a little on to the price to allow for bartering.
I think if you are to look objectively that any and every dog breed has it's list of problems. I think if you look carefully and find a dog with health tested parents, then you can't go too far wrong.
Despite cavaliers being fairly prone as a breed to health issues, I will never be without one...
Lovely dogs. I have them and I have known plenty. Never met one that I didn't like.
One of mine is about 10 now and is just as active as she was as a puppy despite always having a minor heart murmur. Mine have never had health problems except one who has had ear infections once or twice.. But...