Funny, I’ve asked this exact question multiple times, and the answer I always get is that you just get fucked. You pay full price for the first vehicle, and then 90% of full price for every extra one.
Maybe it’s Georgia? I carry the minimum for the state (Liability + Collision), but then bump the Liability because I now live in a high cost of living area. So if I’m in a wreck, I get nothing, but the other guy gets a nice check for his car and his medical bills are covered.
My car insurance would be WAY cheaper for me to just drive one of my shitboxes 100% of the time than it is to split my mileage across four vehicles. But I make it up by being pretty handy. When one of them breaks, I have multiple other cars to drive while I wait for parts in the mail. So I’d still say I come out slightly ahead as far as costs go, but most of my ‘savings’ go straight to the insurance company.