I agree that one should calculate cost per mile on the basis on status miles.
The cost will also depend on continental runs vs transoceanic. I have not gone accross the ocean so I usually compare the cost of my runs with continental runs only.

That being said, in the US the longest flight would be roughly 5500 miles (SEA-FLL). If the fare is $200 (a cheap fare but not the cheapest) then the cost per mile is 3.6 cents. That is my usual gold standard.

If you are very clever (more than me) and you have lots of time to find the right fare and airport sequence and you are flexible with the times you can travel (which I am not) then you may be able to had mileage (SEA-BOS-FLL lets say 7500 miles) then the cost could go down to 2.7 cents.

So, for me 3.5 cents/miles is a good run, 2.5 cents/miles is an excellent run.

Feel free to prove me wrong (there are all sorts of exceptions), but I a see myself as an average MRer only.