We inherited our first two cats from ds's parents. We mistakenly thought that cat jabs were only given in kittenhood, and boosters were not required. We soon learned that this was not the case when one of them got cat flu, was very ill, and then died. It was horrible knowing that it was totally avoidable. All future cats have had their annual jab and checkup. Our next two cats got FLV, which can also be vaccinated against, but our local vet said was not much around in our area. Both cats were indoors/outdoors cats, and there are a few cats in the area, including the cat upstairs, who also died from FLV. We don't know which cat gave it to the other, but it was really horrible, as they were all really quite ill before having to be put down.
I think it is just one of those things you have to budget for with a cat, and 80 pounds once a year is not that much. Here they don't let you adopt unless you have thought through all the costs, including vet fees.
For a completely indoor cat the risk is probably very low, but I am not sure how you keep your cat in and all other cats out in the summer. Certainly our rehoming centre look for this sort of home for immunocompromised cats, who will get very ill if exposed to any infections (this is what killed out FLV cats).