"I bought one way tickets from Cleveland to Amsterdam via Rekyjavik on WOW Air. They were $150 each plus around $100 in bag fees. Total cost for 3 was $560. Does anyone have advice on how to fly back from Europe to either Cleveland or Pittsburgh inexpensively?"
You know the old joke about anesthesia, that the hard part isn't putting someone to sleep, it's waking them up? The problem with a cheap one-way fare is, how cheap will it be to return?
In general, the "legacy carriers" (Delta, British Air, Iberia, etc) charge a LOT more for transatlantic one ways than round trips. But the "budget carriers" usually don't. So, in addition to WOW, look at Icelandair, Norwegian Air, and Aer Lingus. But you can only go where they go, so you may have to add on a flight to Cleveland or Pittsburgh separately. If you do this, be SURE to allow enough time to get through immigration and customs on arrival at your first US airport.
"What airports are usually most affordable to fly from?"
Alas, there's no consistency. You just have to check all options. Case in point: in March 2017, I was looking at fares from New York for May 2017. JFK to Zurich was $350 (not a typo - three hundred and fifty dollars), and yes, that was nonstop, round trip, included all taxes and fees, and even gave me a choice of legacy carriers (American Airlines or Swiss Airways). But JFK to Geneva, for the sames dates, was about $1200 nonstop, $650 with a very long layover in Casablanca (on Royal Air Maroc), or $700 with a decent connection. Why was Zurich so cheap? Why was Geneva so much more expensive than Zurich? No one knows except the airlines, and they're not telling.
