I never get the insurance, its just a way to get more of your money for no reason. I get covered under my amex credit card "Car Rental Loss and Damage Insurance" with no deductible and my own insurance policy. My personal liability,comprehensive and collision apply to any rental car with my same deductible.
If you want really good coverage to ensure no out of pocket fees if you get in an accident you can call your insurance company to add Use of Non Owned Car-Physical damage (UNOC) coverage for like $5 a month and cancel it after your trip.
There are much better deals for full insurance on a rental that covers all fees without going through the rental company if you are renting longer than 2 days. The amex insurance is better and cheaper.
As for the different between full and liablity on my 04 corolla, it was less than $15 a month or $180 a year with state farm. -- ...brought to you by Carl's Jr.