One of the things that's not entirely obvious to everyone is that the cost of laying track (ties, rail, etc) is almost immaterial to the cost of reconstructing an out-of-use line. It's practically a rounding error. Same with signalling, once you have a standard signalling system to use. The vast bulk of the cost is typically in civil engineering: earthworks, cuts, fills, subgrade and grade, drainage, bridges, tunnels, grade crossings, etc.
Now, one of the problems with the Phoenix West line is that, outside UP (who keep their information secret) nobody has done a proper evaluation of the state of the civil engineering along the line in many years. We could be lucky and it could be in great shape. Or all the bridges could need complete replacement and the roadbed could be undermined by waterflow. This makes it totally impossible to estimate the cost: someone would need to actually go and do a structural evaluation along the entire line to get a real cost estimate it. But I expect that the lowball cost numbers assumed everything was great, and the high-end cost numbers assumed everything was wrecked. Only actual inspections will tell you the real cost.