I'm not sure I follow the OP. It's fairly common knowledge that a return flight from Europe to the U.S. will have a different price to a return flight from the U.S. to Europe. There's no reason for it to be any different with low cost carriers offering one-ways.

If booking two one-ways works out better, then do it. That has long been the case with booking reward flights using Avios since ex-US surcharges are higher.