Originally Posted by xliioper

They are not really meant to be comparable. Pay with Miles is for revenue fares (not awards) where you pay for part or all of revenue fares with miles at 1 CPM. Miles+Cash is used on award bookings (rather than revenue fares) and is only really useful if you are short miles for a particular award level and the amount you are buying is close to the amount of miles you need. Unlike Pay with Miles, the amount of miles you purchase with Miles+Cash is fixed by DL and not variable. You are paying is roughly 1.9 CPM for the miles in the Miles+Cash case (which is clearly worse than the 1 CPM that DL values them for Pay with Miles bookings). However, 1.9 CPM is less than the Buy Miles option where you are paying 3.5 CPM.

The award levels set by DL are not based on fixed formulas based on revenue fare pricing. However, it is generally within a fairly close range (particularly for US/Canada awards -- awards out of Mexico/Caribbean can still vary dramatically from revenue fare pricing). There may be cases where Miles+Cash on an award ticket works out better (but again, it rarely makes sense if you already have the miles to cover the award), and others like the above where Pay with Miles on revenue fares will be better. Also note that Miles+Cash is available to everyone, while Pay with Miles is only available to DL Amex cardholders.

Note that they don't apply the 15% Amex discount to Miles+Cash tickets (even though they are based on award pricing). In below example, you are paying $290 for 15K miles to reduce award price from non-discounted 130K miles to 115K. As an Amex cardholder, it will likely never be the case where you can come out ahead with Miles+Cash vs. a straight-up mileage award. However, in example below, both options come out better than Pay with Miles.