largest discrepancy between o/w and roundtrip prices
 

Travelling to MAJ (Majuro, Marshall Islands) is very expensive however you look at it , i.e. HNL-MAJ $1600 roundtrip, $940 oneway.

Interestingly, NRT-MAJ roundtrip is a bargain at $943 (H class) but the cheapest NRT-MAJ oneway I have found is $1921 (Y class - same dates). Tried a few dates in November and it was pretty much like this throughout. I don't normally like throwaway tickets but I only need a oneway on this trip. They must be trying to attract Japanese holiday traffic to MAJ (good luck with that)

What's the biggest encouragement UA has given you to think about skipping a return leg? Any other 'bargains' to get to MAJ?

Miles. That's what I would do if I had to do a one-way in your situation.

I have seen IAH-LGA Round Trip for $340 and half of the exact same trip price as $800 One Way. It's one of those mixed business/pleasure situations requiring me to travel one way at my own expense that makes me happy UA allows one way award tickets.

As a percentage or in absolute dollars?

IAH-WAS can easily be $750 one way, $250 RT if the RT covers the right days.

don't use your MP number and skip out on the return leg. or use your MP number and cancel the remainder. this will only cause you issues if you book as part of a company that uses a travel agent (they can get penalized) or if you yourself are a travel agent ;)