I used to be a veterinary technician at a cat only clinic for a 4 years. There are a number of reasons to have teeth cleaned in animals of any kind. Plaque causes tooth decay just like in people. Plaque in cats can eventually lead to kindey failure once it spreads into the blood stream. The price your veterinarian quoted sounds about right but ask in advance about extras. Ask if you pet will get an injectible or inhaled anesthetic. Injectibles filter thru the animals organs where inhaled do not. Isoflourine or sevoflorine are usualy used. They wear off much quicker and are much safer for elderly animals. If your cat is over 10 ask if the price includes blood test prior to anesthetics to check for any possible issues. Also ask if there are extractions how much extra will that cost and looking at teeth does he foresee that may be necessary. And many techs forget to tell clients - make sure your cat gets no food or water after midnight the prior day. It's hard casue cats will drive you crazy but a cat that vomits during the procedure could easiy die.
If you have any concerns at all you should be able to discuss them with you vet yet I've seem many that don't feel the need to discuss with owners like we would people. If your vet is like that - get a new one. Your vet may be suggesting to do now as once the get 12+ anesthetic procedures are not recommended except for urgent care.
Sorry my answer was so long. I think dental care in animals is as important as people.