Dan-
Do you have a formula or a way you do the calculation?
To illustrate. You are a non-elite with 100,000 CO miles thanks to AMEX.
You want to fly CLE-LAX nonstop which is $460. You would earn 4,100 miles for that.
If you don't redeem miles you will now have 104,100 miles and your wallet would be lighter by $460.
If you redeem 25,000 miles plus $5 tax you will now have 75,000 miles and your wallet would be lighter by $5.
The mileage spread is 29,100 miles. The cash spread is $455.
455/29,100=1.56 cents per mile.
That's my basic "buy or redeem" formula.
Thus for JFK-LAX-LIH the mileage spread is 70,000 miles. The cash spread is $1,290.
1290/70000=1.84cpm.