Let's say your cost is 25 cents/mile so you'd "net" 12 cents/mile.
The cost per mile is what I'm really trying to figure out here. I'm looking for some math help on calculating the depreciation/cost to operate. I am familiar with the logic behind the other calculations. There are a lot of other factors in play here that I'm considering.
How much life do you think is left in your Corolla?
I think it's good for a while. I expect to drive it for at least another half-decade.
The cost to operate a car is composed of maintenance/repairs, fuel, insurance, depreciation, and registration/etc. Here are my numbers for two cars over about 60,000 combined miles.
Maintenance/repairs: $0.03-$0.06/mile This is with me doing my own work; probably about double it for me if you farm that out.
Fuel: $0.07-$0.15/mile One car gets about 20MPG, the other 45MPG. Gas is cheap right now ($2.09 near me), so both numbers are falling as more miles are piled on.
Insurance: $0.02-0.03/mile Largely outside of your control; just make sure you're not overpaying.
Depreciation: $0.00-$0.01/mile Cheap cars bought for almost nothing, still worth a similar almost nothing.
Registration/etc.: $0.01/mile Basically out of your hands.
Total: $0.18-0.22/mile
Being a bit conservative, here's my best guess at your numbers:
Maintenance/repairs: $0.06/mile Assuming a reasonably reliable car (Toyota) but without doing your own work. Check your records if you have them to calculate a real number.
Fuel: $0.06/mile $2.09 / 38MPG
Insurance: $0.03/mile Made up number, varies with person/location.
Depreciation: $0.04/mile? Just spitballing here with a guess that the car is worth about $3500-4000 and has 100,000 miles of life left in it. Your acquisition cost may have been $0, but deprecation is simply current value over expected lifespan.
Registration: $0.01/mile Made up number, varies with person/location.
That adds up to about $0.20/mile so my numbers that I threw out in the other post are pretty close. I'd call it $0.25/mile just in case gas decides to double in price again, or your insurance goes up, or other such external factors.