I have found them to be lower most of the time and joined just for that benefit. The prices go up and down. I am currently checking a few times a day for a trip coming up Jan 7. My current cost is $135 for the week and expect to get it lower but it is still lower than what I've found elsewhere.
I originally joined Costco for this and the primary insurance coverage I could get with the AMEX card they were using at the time. I now have a different AMEX card for the insurance but kept the Costco membership for the car rentals. They often send out coupons and I save a few dollars when I use them to shop.