In the same way that basic prices for the rental car itself vary from company to company,
car size to car size, city to city and even location to location within the same city... car rental insurance rates vary as well.
There are no set prices for insurance options for each company... they vary according to the above factors as well and will depend on which type of coverage you are looking at:
See Car Rental Insurance Overview for more
info on different types of coverage.
In order to compare rental car insurance rates, you would need to research pricing for each individual company, as you've done, and compare the cost for both the base rate of the car, and then the price if you select your preferred insurance options. The price for optional car rental insurance will vary, even withing the same company, by location and often depend on the size car or vehicle that you rent.
Most car rental company websites (links open in new windows) Alamo.com , Avis.com , Budget.com , Dollar.com , Enterprise.com , Nationalcar.com and Thrifty.com for example, allow you to price various optional insurance coverages when checking rates.
Make sure to compare rental car insurance rates from mulitiple car rental companies in addition to the cost of the rental car in order to determine which company offers the best bottom line price for the car plus coverage!