Did your vet do x-rays of the jaws/skull? Have you seen the x-rays?
Dental problems are common in bunnies especially as they get old, because the bones in the skull gets thinner with age, and this can make the teeth misaligned. Bunny teeth grow all the bunny’s life, and needs constant wear. The molars are kept in shape by the teeth in the upper jaw grinding against the teeth in the lower jaw, in a sideways motion when the bunny eats tough food like hay.
If your vets suggest surgery, I’m guessing they want to pull the molars out. That’s done if the dental roots are in a terrible shape or if there are tooth-root abscesses etc.
Many bunnies whose teeth are not worn properly develop molar spurs, ie enamel protrusions from the teeth. These can be ground or burred down. This should not cost 500 dollars. The bunny has to be anesthetized. The cost would be more like 100 dollars. If your vet isn’t planning on pulling the teeth but is charging 500 dollars for a molar burr, find another vet.
If nothing is done, the teeth problem will get worse with time because the teeth never stops growing. Apple may not have everything a bunny needs in the long run, but you can try getting him to eat Critical Care or pellet mash, served on a plate (pellets + water + f ex fruit purée for babies or canned unsweetened pumpkin), and if you absolutely have to, you can support-feed him by syringe, so nutrition is not the primary problem. The main problem is that the dental issues won’t go away but get worse.