I think it is not so much population of the metro area, but butts in seats. At the Stallions' last game, they had 5,500 folks in the stands. If 1% of people in metro Boise attend a game and 0.1% of people in Birmingham attend, which one puts more butts in seats?
On the other hand, TV revenue is, I think, national (i.e. ESPN and Fox Sports paid to cover games nationwide).
My big concerns is playing football in Boise in March. (First game is scheduled for ~7 March). It could be pleasant in Idaho. Or it could be snowing. Snow might put a damper on attendance.