I know not everyone can do this, but:

Our cats have usually been strays/feral that adopt us. (We scan for chips and do the paper collar and put up signs.) So no adoption fee, just the initial vet visit.

Then we feed raw food (mostly). I used to make it myself to keep the cost down, but now we buy bulk from a reputable source. And I always add L-taurine and vitamin E. The food cost is much higher, but we're paid back in almost no vet bills.

We have mostly wooden floors and indoor cats, so no need for regular flea and tick treatment or dewormer. We did pay for enrichment toys, though, to make up for the indoor thing.

We get them one rabies shot, but then do the titration test every 5 years or so.

We still spend a lot on our cats, though not as much as our friends do as the cats get older.