Not all airlines apply silly loadings to one way fares. Some airlines price all legs the same and for a return they just sell you 2 one ways. These can be very useful to 'fill in' multi city trips or do as you are doing, act as the return to a one way reward booking.
WestJet, Norwegian, Icelandair and Tomas Cook spring to mind as likely options for a sensible one way quote. If you aren't in a rush, Icelandair also allow free stopovers of up to 7 days in Iceland. Who doesn't fancy a dip in the Blue Lagoon on the way home?