Originally Posted by
Evan!
Whenever I calculate the cost of miles I only use the incremental amount I might spend to get the bonus... So if I get 9,999 miles but pay $40 more than I would have paid I consider my cost per mile to be $40/9,999 miles = 0.4 cpm.
I agree that you should only count the incremental cost. (If you didn't need the car at all, it's all incremental.)
However, don't forget to only count the incremental miles. For example, suppose you would have gotten 1k miles at the lowest rental cost. That would make your cost per mile $40/(9999-1000) =.44 cpm.
If I could do .44 cpm without significant investment of time (e.g. I was renting a car anyway) I would do that all day.