At the time of publication, there are $230 nonstop, round-trip fares from both New York and Boston to Madrid between September and March 2020 available via Google Flights. Incredibly, nonstop, round-trip tickets from Los Angeles to Madrid between November and January are similarly cheap, at just $310 a pop. We found great fares being offered by several airlines, though most of the best deals appear to be on Iberia, American, and United.
You may need to do a bit of searching on Google Flights (or whatever other sites or apps you typically use to score cheap flights) to track down the cheapest fares. Also, it's possible the insanely good deals could disappear in a matter of hours.
In other words, this may be one of those "book now, figure out later" situations, since it's almost unheard of to find a nonstop, round-trip ticket to Europe for under $250.