Of course these numbers cannot be translated directly to any expansions of the WDW monorail as these costs do include acquiring land and some include large tunnels or traversing large waterways. Also the number of stations, any switching tracks etc... can effect the costs tremendously.
Only Disney can figure out the true cost. Disney does so much construction that the may have their hands in the business. My meaning is, that even if a project comes with a hefty price tag, they may still do it if the company doing the construction is a subsidiary company of Disney.