From op's data,
10,000 - $0.7354
15,000 - $0.5646
20,000 - $0.4944
I just don't see how you get that marginal cost down to $0.20, yes mile n+1 is a little bit cheaper than mile n. and sure, let's pretend that high milage cars used cars sell for the same as low milage cars. so financing is fixed. I just don't see how you can get regular maintenance and gas and insurance down to 20c/mile.