After your cat has gone through its more elaborate set of kitten-shots, the FeLV vaccine should be given once a year. If you adopt an adult cat and don't know if it has been vaccinated for FeLV, start it immediately on its annual shots. If you are unsure if your adult cat needs be vaccinated, talk to your vet. Prices vary between vets, locally and nationally. I surveyed readers of rec.pets.cats and contacted vets around the country and found that prices for the vaccination alone cost between $9 and $30 dollars. The average was $18 (not including the vet-visit charge). It seems that the FeLV test costs about the same, but I didn't do a full-scale survey.