I've never bothered with pet insurance for my dogs or cats - I've never done the math, but I doubt it really pays. Maybe if I couldn't afford reasonable treatments, I would consider it.
Vets are expensive for sure, but some will also do unnecessary stuff. Question everything, get the vet to explain why it needs whatever they're recommending. There are a lot of vaccines in the first 6 months, but after that annual shots and checkups are usually all that's needed. Once cats are older, they can get 2 year rabies shots, which cuts down on the cost further. Of course, if illness crops up, that's more expense, especially since emergencies always seem to happen after hours. Make sure you know where you can go in the middle of the night or weekends.
Keep the emergency number on the fridge, because when you're in a panic, that's the fastest place to find it.
Also, with your cat coming from a shelter, it should be somewhat cheaper than mine were, since they were strays and came with some issues like ear mites and worms.