Is the a particular country in Europe that has less expensive flights back to the USA? I will be travelling all around Europe and can make any counrty my destination to return home.
About the only thing impacting ticket prices is local taxes and landing fees. Some complain that London Heathrow has high taxes and fees. I would pick an airport for convenience and scheduling and not worry about less fees. The savings would be small in an event.
You ask about returns, not round trips so I assume you have a plan for where to start? On many airlines, buying the depart and return on one ticket is cheapest, even if its a multi-city (example fly into London, out of Rome). If you don't want to buy one ticket that way, then look at airlines that sell one way flights to the US for roughly half the cost of a round trip. Not all do.
Offhand, I can think of TAP Air Portugal and Iceland Air. So fly home from Lisbon/Porto on TAP, or Reykjavik on IcelandAir. Both go to multiple US east coast cities. I'm sure there are other airlines as well but those two are my first thoughts.
The busiest route across the Atlantic is between New York and London so pricing can be competitive.
However, Icelandair or Play Airlines (another airline that flies via Reykjavik) probably have the lowest prices.
Level— Iberia's low-cost subsidiary foys from BCN and MAD to the US.
Norse—the revival of Norwegian's low-cost, long-haul, strategy with a less ambitious route network flys to the US out of Oslo, Berlin, and Gatwick.
Some complain that London Heathrow has high taxes and fees.
Looking at various fare options I've reached the conclusion that many airlines have absorbed the high UK fees in order to make their fares competitive. The fees aren't set by Heathrow but by the government so you'll see the same fees at Heathrow, Gatwick, Manchester etc.
The general rule of thumb is to pick an airport that has a number of airlines flying the same route, that serves to make the fares competitive.