It's not guessing; it is public knowledge that banks pay airlines for miles earned on affinity credit cards. I think the exact rate in the contract as well as the details of how it's paid out (ie is it strictly per mile? is there some sort of threshold the bank promises? Does the rate fluctuate depending on volume or other factors? etc) are commerical-in-confidence, but typical guesses are in the 1.5 cent per mile range for typical airline credit cards (not specifically BofA/AS).