Plaque varies greatly in cats as in humans. If it gets too bad it will cause health problem for an elderly cat. So really only a vet can advise.
Some cats clean their won teeth - my pair of (young adult) cats have such beautiful teeth that the vet commented on it. I told him that the one cleans her teeth by biting off bits of cardboard boxes and spitting them out again on the kitchen floor, and that I have previously had a cat or two who did the same. Others don't but you could try offering yours a cardboard box to lie in and see if the edges are bitten.
The other cat apparently brushes his teeth on mouse-skins while devouring his prey - which of couse requites frequent de worming.
In your case, I suggest you ask a couple of other vets what their charges for cleaning cats' teeth are.