My sister who worked at a crooked vet's office, who heard the veterinarian openly discuss ways to bring more money in by upping procedures and charging more. She was in charge of the front desk and the clerical aspects, the veterinarian ran that place like any business and that included charging every customer as much as possible, telling them they needed x y and z done for their animal. This is common practice for vets from what I've heard others tell me and from my own personal experiences as well. I started catching on when every single time I brought my animals to the vet, they "needed" something else.
Nearly $700 for a cat's dental work honestly sounds like just another one of those situations. Cats tend to have plaque naturally since they don't brush their teeth like we do, they used to live in the wild and never once had a dental check up until humans came along. lol they don't naturally have white teeth or perfectly looking dental work, but I can definitely see how a vet would charge for that...

Edit - Now if your cat had an "impacted tooth" or a mouth infection or something equally serious, that's another thing. But plaque and inflammation? That does sound like a gimmick. Basically every animal has plaque, and inflammation simply means reddening of the gums which again is basically what every animal and lots of humans have as well.