Wirelessly posted (BlackBerry8703e/4.1.0 Profile/MIDP-2.0 Configuration/CLDC-1.1 VendorID/104)
There really are so many variables.
Some of the foreign airlines offer one-way awards (including awards on their U.S. Partners) for half the cost of a roundtrip award. Most U.S. carriers do not offer that option. Some foreign carriers charge fewer miles for certain awards on U.S. carriers than the U.S. carriers charge themselves! Some foreign carriers (Virgin Atlantic and bmi come to mind) award more miles for car rentals and hotel stays than their U.S. counterparts do. (And there is no FF excise tax payable when you have your car rental miles posted to a foreign airline.)
Lots to consider.