Delta employee site has travelnet--all the calculations are available there. You make a dummy booking and cancel after you get the price. I think you got lucky on the Madrid trip--you were also supposed to pay US fees/taxes, too.
here's and example of Malaga fees. Some cities have a separate departure tax on the ticket, some you have to pay at the airport prior to check-in. No cost for mileage.
AGP / JFK Spain Passenger Service Charge Liability 10.47
AGP / JFK Animal Health Inspection Tax Payable 5.00
AGP / JFK Immigration Service Fees Payable 7.00