First-hand info regarding the project:
Anticipated re-opening is April 26. To date, 49 bents have been repaired with new piles driven down up to 160'. Nine steel spans have been constructed to replace the worst section. What they discovered was that over the years, maintenance would be done from the closest (east) end to HQ. This work would work proceed west until the money or budget stopped. By the time they got back to it in a year or 2 or 3, some new work would be needed again in the east side before proceeding west. As a result, the west end got neglected.

I won't go into specifics regarding cost so far, but let's just say that if they had the $5.5 million in-hand that has been estimated to replace the bridge east of Colfax, they probably could have replaced the entire Benewah trestle.