Your question asks how much you would be paying for insurance with the information given.
The reason why your answer would be $300 is because that's the premium that you would be paying for. The "premium" means the amount you're paying for coverage. The premium could have different coverages that make up the price. The insurance would cover the liabilities that you might have.
People tend to get confused with deductibles. You don't pay monthly for deductibles. Deductibles are a payment that someone needs to pay before an insurance company starts paying for your needs that your coverage provides. For example, if I brake a bone, I would first pay the $500 deductible before the Insurance company starts covering my costs. This is the ensure that the insurance company gets some type of money before they start helping you.