How much does auto insurance cost for a 16-year-old girl?

You cannot estimate premiums in a forum like this. There are far too many factors that go into rating insurance. Some of these are State, County, zip code, driver age, vehicle, use of vehicle, driver's record, coverage, and many more factors. One insurance company I represent has 44 classifications just for the driver. I recommend that you insure her on the family insurance policy. If the rate is not acceptable call independent insurance agents, find one you trust, and get them to figure rates for you. Independent Agents represent several insurance companies and can shop for the best rates and coverage for you.