Erie, CO
The halfway point between Spokane, Washington and Dallas, Oregon is between Arlington, Oregon and Boardman, Oregon. The total mileage is 413 miles, so halfway is 206.5 miles which puts in between Arlington and Boardman. On I-84 there is an exit ramp to Heppner Hwy #74 which is 206 miles from Dallas, Oregon and 207 miles from Spokane, a perfect halfway point.
lincoln, nebraska
Taylor, Idaho is the halfway point.
North Dakota
Meridian, Idaho
salt lake city
La Grande, OR
It is in Meridian, Idaho
Gretna, NE
No. Buffalo are a popular pet in the state of Idaho, and people from all over the world come to see the "Buffalo City" Meridian, Idaho.
Pendleton, Oregon, seems to be about halfway between Boise and Tacoma. The Tri-Cities are nearby the halfway point, and they are a larger metro area.
The halfway point is basically the area where Nebraska, Colorado and Wyoming meet. This is not an exact answer, but in relative terms this is the halfway point between Salmon, Idaho and Kansas City, Missouri. If your looking for a town, then I would say the capitol city of Wyoming (Cheyenne) is pretty much as close as your going to get to a city halfway between Salmon and K.C.