Whenever my cats have gotten abcesses (usually from fights), the only successful outcome has been letting the vet flush the wound and following up with antibiotics. If your vet is too expensive, find a farm vet. Even if you live in the city, the cost difference will be worth the drive. Farm vets are usually a lot less expensive than the townie vets, and they have a realistic view of what you can and can't spend on your barn cats.