I honestly still believe Disney could do this MUCH cheaper than Vegas or Dubai. Let me preface the following logic and observations by saying this is just my opinion and I have absolutely zero evidence to support my beliefs...
I'm pretty familiar with the Vegas system. That system was built in some extremely tight construction corridors (middle of major roads, going between close buildings, etc.) and also uses multiple massive straddle bents to cross major roads that are really wide. They also needed easements for the land.
Disney owns the land they would be building on. Also, theoretically the location should provide easier access for construction equipment since it isn't really an urban setting like Las Vegas, causing fewer impediments to the construction. From what I understand the cost of the land, as well as the location (urban, rural, etc.) have a significant impact to the costs of building.
Another observation I had about the Vegas system concerns the stations. The stations on the Vegas system are massive compared to what is currently at WDW. The stations had to be fairly expensive in comparison. I have to assume this also contributed to the high cost per mile.
s8film40, was the plan you saw similar to the map you created and posted previously? If not, I'd love to see the plan you speak of if that's at all possible. It sounds interesting.