Oneworld member Iberia has lowered the prices for most of its nonstop flights between Madrid and the USA. Prices start at €223 for flights to Miami and New York. Even if you add a piece of luggage for €60-100, you’d still be getting a great price. You can fly practically anytime except during the summer and Christmas vacations.
There’s very good availability from November 2021 through August 2022, the lone exception being the two week s before and after Christmas.
You can fly to the following destinations for less than €300: