So I'm the OP of this thread:
https://www.dcurbanmom.com/jforum/posts/list/1299635.page
Basically the same issue, though in my case it's the time/energy more than the money that is the problem.
We got the UTI treated. She's had another issue since then.
What I've decided for all this little stuff is five strikes. Once the fifth issue pops up, then it's time to call Lap of Love. Obviously, any bigger issue may end this sooner, but my concern, like you, is the never ending small stuff once they reach old age (my girl is 15). If she has a stretch of a year or so with no issues, I'd start the count over again, but that seems unlikely at this point. Cats, like people, start to deteriorate as they get older.
Maybe that's not fair to the cat - I'm sure there are people who will think I'm awful. But she doesn't like all these vet visits and medicines and pokes and prodding either. She's lived a good life, she's still living a good life, but five is my limit. It's also made these issues more manageable, knowing it's not never-ending and there's no way I'm doing this once a month for a year.
Maybe your limit is more of a dollar figure? But having a limit helps. You've done will by this cat - you don't need to go into debt to help your cat.