Let's assume that the price difference between Tango and Flex is $50 per segment (although, recently, the price difference is in the $100-$150 per segment range for NA travel).

$50/segment x 21 segments = $1050.

$1050/25000 miles = $0.042/mile

$0.03/mile to buy top-up miles on ae.com

(of course this is a simple calculation and does not take into account the full vs 25% award miles accruing with these fares -- but the calculation is based on the lowest per segment price difference too)