I don't have a set formula here, but I do have it on the two vehicles that would cost several thousand to replace (my Sienna & Highlander). When I took over ownership of the 98 Camry with 260K miles on it, the insurance company, in case of accident, was only going to give me $1500, and I would save $300/yr by not having comp/collision on the car. So I declined comprehensive on the Camry.
For me the issue really wasn't a percentage of original value--it was more a matter of, having to shell out big bucks to replace those two cars (I probably wouldn't be comfortable buying the same model year used somewhere, so replacing them would mean I'd shell out a lot more money for late model year cars). So I definitely wanted something to bring the cost of the new car down if I were to total one of those other two cars--the payment from the insurance company. But a policy that would get me $1500 or so for the Camry, was just not that big a deal, so I was willing to roll the dice and save $300 a year on that. So I guess part of it, was how much I could save by not paying for comprehensive.