I always wonder where the numbers come from when people say 1 million a mile, 18 million a mile etc...
Projects like this would be so site dependent it is hard to get a really good guess at how much it would really cost. One of the steps in estimating a project is to first see how much similar projects have cost to complete, such as:
Naha, Okinawa - 7.8 miles @ 533 million = 68 million per mile
Kuala Lumpur - 5.3 miles @311 million = 59 million per mile
Las Vegas, initial - 3.6 miles @ 352 million = 98 million per mile
Las Vegas, extension - 3.1 @ 337 million = 109 million per mile
Seattle Green Line - 14 miles @1.25 billion = 87 million per mile
Of course some of these systems have undergound sections or traversed large waterways which will effect the overall cost per mile. However the numbers that are always tossed around do not even appear to be close to the target.
That being said I would love it if they expanded the monorail, it is a part of the Disney World experience for me and one that I enjoy, whether it is feasible or not to expand the system.