I think dtremit is right, when you get a cheap rate you're basically being paid to park the car for the weekend. Nice work if you can get it.
Rental cars depreciate with miles, not age, since they do so many more miles per month on average than a private vehicle. If the company buys a car for $20,000 and then offloads it after 35,000 miles for $13,000 then you're depreciating it at twenty cents per mile, so your $15 a day becomes drastically uneconomical if you start doing more than 75 miles a day.