Calculating Mileage Mall/E-Shopping Points and Promotions
 

I've been wondering how to calculate the value for e-shopping purchases.

It sounds simple: buy some stuff online and get some miles.
But how do you calculate which things are worth buying online vs. offline, and at mileage malls vs. not?

Factors include
-Miles earned per $ spent
-Taxes, online vs. offline
-Shipping, online vs. offline
-Shipping vs. Free shipping (from certain stores, for certain purchases)
-Special promotions (from certain mall sponsors)
-Credit card that would generate 2-miles-per-$ at certain stores
-Convenience vs. inconvenience of purchasing online/offline
etc...

I can post specific example if anyone else is interested in this.

P.S. If there are existing threads on this I would be happy to post there... I ran some searches on e-shopping/mileage malls + calculations/calculating + promotions, but didn't see appropriate.