Looking back on 2012, here's what I paid for car share programs.
eGo CarShare (Denver/Boulder's version of ZipCar)
Summary: 222 miles driven, $405: $1.82 per mile
I use eGo in my home town instead of buying/owning another car; you can see that it's a screaming deal vs owning a car. We don't pay for insurance or gas or garaging or maintenance for eGo. I will note that my office is a walk from my house. I would probably buy another car if my work commute couldn't easily be done on foot or bike or bus.
Summary: 49 miles driven, $84.09 (the $60 was an annual membership fee): $1.72 per mile
I use ZipCar when I'm staying in SF and have meetings either down on the Peninsula or North of the City. Renting a car is a) more expensive b) airport inefficient (you have to deal w/ the rental car center location and process) and c) leaves you stuck dealing with a car for your trip. I generally do a ton of walking to/fro meetings. Dealing with cars in cities is obviously a major pain because parking is so hard. Find a spot... figure out how, when, where to pay for it... blech!
I thought I used ZipCar more than I do. Walking and Uber undoubtedly prevent me from using ZipCar more.
The amount I spend each year on Uber, cabs, and airport car service would certainly dwarf all of the above, but that's another post.