We've been able to get very good one-way fares on Iceland Air the last couple TA cruises we have done. While there are rules against booking a roundtrip ticket with the intention of flying only one way, it is difficult for the airlines to prove that was your intention when booking it - plans change all the time. And rarely do they come back after the fare difference -- I have known them to go after a
Travel Agency that I worked at for it and have seen them attempt to rescind frequent flyer miles for it. But those are far and few between. So if you are going to do it, don't get a
Travel Agent involved, try not to use a carrier you have frequent flyer miles with and have a story as to why you will be using the other half. (One year when we booked roundtrip Barcelona flights - from Barcelona in spring and back to Barcelona in fall; we were questioned at the airport when we were checking in why we were coming back in the fall. The true answer was that we just completed the EBTA and were coming back to do the WBTA. So since we did use the back half of the ticket, I don't know what they would have done if we didn't have a quick answer for their question and then didn't use the ticket.)