I have a non-owners through Travelers. If I remember correctly, it’s less than $200/year. These policies generally don’t cover collision, so you’ll want to make sure you have some additional coverage for that. Regular credit card coverage doesn’t always suffice because it’s not primary insurance. I use AmEx premium rental protection. It’s $20 for the duration of your rental (even for multiple days). Between the two, I’m basically covered no matter what. It’s probably overkill, but better safe than sorry. The non-owners also supplements Zipcar’s insurance. Keep in mind that Zipcar is providing that coverage more for them than they are for you. If I recall, it’s basically the legal minimum.
Call Kristen Mentzer at Alliance Insurance 202-207-1085. (I’m in the process of switching my homeowners from AllState to Travelers and the savings more than covers the auto non-owners policy.) They are in downtown DC.
Disclaimer: I’m not an insurance expert–just someone paranoid about losing everything if I get into an accident.