A reasonably intelligent person would take somewhat dated info to try and make a judgement on things to come. Pretty sure the electricity used for the draglines hasn't gotten any cheaper because the companies making and selling the juice have asked for and been granted a couple rate increases in the interim. Even though the mine buys power from the same plant they are supplying coal to, they don't get a special rate.
As mentioned, the coal continues to get harder to mine and I seriously doubt the new seams they are mining are easier or cheaper to produce from.
Diesel fuel used to transport coal from shovel to conveyor hasn't gotten cheaper, nor have the diesel machines used for transport.
Ammonium nitrate hasn't decreased in price either. Coal energy isn't getting any cheaper and more and more people are choosing not to use it. Big problem for Colstrip.