Check out this comparison by Ari Ofsevit of the costs of various transit expansion projects around the country. When you look at the overall cost per mile, the Green Line extension looks irrationally high. Important note: Every project is unique, so it’s very difficult to compare these kinds of costs apples to apples. But it’s worth a look.
In addition, read this great piece from The Transport Politic, also covering light rail construction costs and comparing multiple projects. When the Green Line cost estimate was at $700 million to $1 billion, it was in line with other US light rail projects being designed (in costs per mile). But still, like most all US projects, more expensive than similar European light rail projects. Now, at $2 to $3+ billion, the GLX is in the per-mile cost category for a subway system, not surface rail!