If a US airline were to offer a one-way award, it would probably go something like this:
PlanAhead roundtrip: 25,000 miles
PlanAhead one-way: 87,643 miles
Anytime roundtrip: 40,000 miles
Anytime one-way: 134,523 miles
Of course, it would come with a bunch of nasty verbiage about how sinful it is to throw-away the return award leg.