A car is going to lose some value every month no matter how you finance it or pay cash. So it's safe to assume a few hundred a month for depreciation for a newer model car like Zipcar provides.
Rule of thumb is on average 15% of the value per year, or roughly 1.2% each month. $300/mo would be depreciation on a car worth $25000, which is in line with the value of a new-ish Honda Accord
That's not how depreciation works. If the rule of thumb is 15% per year, then it is 15% off of the remaining value, meaning a new-ish Honda Accord would be worth about $9,400 after six years (about $216/mo).
Whereas if you take 15% of $25k off every year, the car would be worth nothing after about six and a half years. We know that's not true.
It depends on whether you are using the straight line or declining balance method. In this particular context, the 15-20% rule of thumb for cars does refer to declining balance, which does reflect the market value of cars more closely.
If it were straight line though, you could just have a predetermined residual value for a specific useful life of the asset, and then depreciate the percentage of the difference from the original value and the residual value each year. You can also easily have depreciation where the residual value is less than the fair market value at the end of the asset's useful life or stated period though. You can never perfectly predict the residual value after all, but that's what capital recovery, or gain on disposal are used to describe.
That depends on how you depreciate. Straight line would give you a fixed monthly amount. Financial types prefer declining balance (% loss per year) as it front-loads the decrease in value (providing front-loaded tax benefits).
