Elk City, OK is pretty close to half-way.
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.
Monticello, UT
Fairfax, MO
La Grande, OR
Price, UT
The halfway point is Gandy, UT.
Forsyth, MT
Boise, Texas
According to Meetways.com, the halfway point between Boise, Idaho and Sioux Falls, South Dakota would be on US Highway 14 in the Bighorn National Forest near Dayton, Wyoming.
If driving, the halfway point is in Salt Lake City, Utah, with about 341 miles to Boise and 341 miles to Montrose. As the bird flies, the halfway point is just north of Salt Lake City, around Layton or Bountiful.
Penrose, CO
It is roughly 1700 miles from Boise, ID to Chicago, IL. The halfway point between these two cities would be 850 miles, which would put you around Sidney, Nebraska.