Answer:
You should use the discount coupon to pay for the Chicago-Miami trip. Not considering the personal motivations for the trip, the coupon is worth $500. The cost of flying is $600, so you will only pay $100 yourself. You will be spending $900 + $1000 = $1,000 in total.
The opportunity cost of using the coupon is $350 (the cost of the round trip to Atlanta). Even if you add the $350 to the $1,000 expense, the total is $1,350, less than your $1,400 maximum budget.