WOW Air has $55 one way fares to Europe from Los Angeles, San Francisco and Miami for travel in June 2017. The one-way fare back to the USA is many times higher. You will need an alternative ticket home using miles to get a real round trip deal.
There are only a few dates for each route and limited tickets.
Google Flights calendar offers a quick view of low fare dates. Links below are Google Flights date with $55 fares at time of posting.
Additional low fares to even more destinations like Cork, Ireland; Edinburgh, Scotland, Brussels and Frankfurt are available from Miami.
I booked a one way ticket for $150 on Iberia’s Level for travel SFO to Barcelona this month. I will fly home on a round trip ticket from Stockholm flying British Airways with a return flight next December for $470.
WOW Air is bare bones flying with no inflight entertainment and charges for everything. I flew WOW Air twice in January and April on $70 one way tickets SFO to Copenhagen. My flights were $70 all-in traveling with no self-selected seat assignment or paid bags. I traveled with one small backpack in winter to Iceland and Scandinavia and survived. Summer travel on WOW with a small carry-on for free is certainly manageable.
Getting a deal back to the USA is the real challenge.