Two camps. One just runs on the insurance as if it was stock. And if they ever need their policy, they are going to be financially bankrupted.

The other camp is actually insured and yes it’s ridiculously expensive.

For example if you just have a new bumper or body kit with standard insurance, and someone tbones you, you’re standard insurance will probably pay out but will assume your car is worth less than stock because of modifications.

But if you have that body kit, and you run over a pedestrian, you’re going to personally be responsible because the insurance will say you modified the crash tested crumple and pedestrian zones on your vehicle. That violets the contract you have. Not only will they not pay, they’ll drop your policy. Then you’ll have to pay the millions for paralyzing little Timmy.

Best case scenario for standard insurance is you buy a $500 Fiero, do $50k of modifications to make it awesome. Someone crashes into you, and they total your car for $500. That’s the value of the vehicle.

As another data point one of my collector cars is a low mileage 1980’s vintage. It’s worth 6 figures and while the past few years is a garage queen, I do actually drive it on the road. I have special insurance for it that will pay to “restore” the vehicle in the case of an accident. You can’t buy parts new, and 99% of mechanics won’t even touch it. So anything in a shop is going to be ridiculously expensive. That’s what my insurance pays for. To make it back to current condition after an issue. I pay 200x to insure the vehicle than I do to register it on the road with the government.