Interesting process you're going through. I've tried similar things in order to justify sometimes unnecessary bike expenditures to my wife, with only marginal success.

I guess any calculation is inherently theoretical. You could figure out what service should have been performed between 70 and 100k miles on your car, figure out cost for parts and labor, and then factor that in. But, in reality, you don't know what other costs might arise other than anticipated service. All you can do is guess. Certainly you can figure the cost of gas quite objectively.