Barcelona or Madrid — $338+ (RT, bags extra) Nov – May Departing from ORD,DFW,IAH,LGA,EWR,PHL

You can use google flights to look up these deals.
These routes are on American, United, or Delta basic economy which does not include seat selection or a checked bag. A checked bag costs $60 each way.
TO: Barcelona (BCN)
Dallas (DFW) – $353
Newark (EWR) – $331
TO: Madrid (MAD)
Chicago (ORD) – $338
Dallas (DFW) – $358
Houston (IAH) – $338
New York City (LGA) – $380
Newark (EWR) – $352
Philadelphia (PHL) – $358
The Google Flights calendar is not showing up consistently. You may have to do some guess and checking with dates in the availability that we provided. For instance: a sample date like December 2-11 may show up as $474 in the calendar view, but when you select those actual dates, the fare drops to $338. We found that many of the fares showing in the $400s in Google Flights calendar tend to drop in the $300s when clicked through.
http://www.google.com/flights/