Generally what they do is charge you all taxes, fees, YQ and YR that would appear on the exact revenue booking. Try using matrix.itasoftware.com, search round-trip, origin JFK, destination ZAG, on your dates. Then in Advanced Routing Codes specify F:YYnnnn F:YYnnnn where YY is your carrier and nnnn is the flight number.

Select the price and look at the breakdown. Anything in the XT (combined taxes and fees -- that is everything other than your fare/basis) is what you'd pay on your trip.

You should be able to get away with ~$200 in taxes and fees if you limit your travel to United or US Air. At least this is how things work with the other Star Alliance partners.

Also if you have BA miles consider flying Aer Lingus JFK-DUB/SNN-ZAG (if that's a valid routing and there's space -- check availability using the United website). They will not charge you fuel surcharges on EI flights. [Edit: EI doesn't fly into ZAG. That would only get you as far as DUB/SNN]

Best of luck!