The insurance company will go by whatever the book value is on your car and that's the max you'll ever get out of them. In my case I'm looking at maybe $500 so I don't bother with collision insurance, I'd quickly be paying out more in insurance than the car is worth. You have to run the numbers to see if it's worth it in your situation.

I put collision coverage on my 1999 Ford Ranger, even though it has 270,000 miles on it. I had the transmission rebuilt at 250k and did this to protect my investment. My truck looks pretty good for being 22 years old and in the rust belt, and the engine runs like there's no tomorrow. I saved the receipts, so if I ever end up in the unfortunate situation of having a totaled truck, I'll be able to haggle with, and prove to someone's insurance company that I put $2800 into that transmission. The shop didn't do a good job on the rebuild, and I may install a remanufactured unit myself. If you plan on doing serious work like this to a vehicle, definitely put collision on it, and save your receipts. Years ago my mom had put $800 into her clutch and a few months later, the car got totaled. The insurance company gave her the car's value, plus the value of the work she had done because she produced receipts.

I'm also insuring a 2017 Mustang and a 1979 Catalina on the same policy. Adding both of the old cars actually dropped my premiums around $25 vs just having the Mustang.   

Sometimes paying for basic insurance on a car may not be the best idea. It's better to be covered than not covered for collision coverage. If you're worried about the value of your car right now, sell it and try to make money off it.