It saves me roughly $350/year on each car. So in 15 years it has saved me roughly $10,500. I would have had to have 21 accidents in 15 years to justify the extra cost. I don't know how that's not cost effective?
I'm not saying what you are doing is wrong, not by any means. I am surprised that not having any accidents in 15 years means a difference of 350 a year in comp coverage between a 500 and 1000 deductible. It is usually much less than that in premium difference (from my experience), which I would estimate at being around 120 a year difference between the two deductibles (per car - again, based on my experience in the industry).
Obviously, if you have an excellent driving history and/or can self insure for the deductible then carrying high deductibles is an excellent choice. Not a lot of people can say that, thus the difference in premium from 500 to 1000 is typically minimal for those people.