I don't think cost alone is the issue. Word is they spent $1 billion on the whole MyMagic+ system. That would translate to 1,000 milies worth of monorail track at the price you mentioned. If they had really, really wanted to, they could have built more tracks around WDW as was originally planned, and then some.
The biggest difference between those two is that unlike MyMagic+, the monorails generate no income but still have their expenditures.
It doesn't work like that. The cost of rides, resorts, tickets, and transportation don't all come from the same place. The pool MyMagic plus comes from is separate from what monorails would come from. A company a big as Disney is going to have different budgets for different things, and while it could be argued that research and development could be an area they both fit in...past the development stage, that ends.
And I wouldn't say monorails generate no income. In fact, I would say the both of them, MyMagic+ and monorails are in the same boat there. Neither directly draw income, but they both draw in more park traffic, and are expected to pay for themselves over the long term. A monorail would decrease transportation expenses for Disney, likely increase resort costs (if it connected to any more resorts), and make not only a Disney trip more attractive, but make staying at any resorts it now connects to more attractive.
I could be wrong on the part of resort costs, though. I know someone said something about that in another post. I can't remember if they said it's been confirmed that the resorts DO work in the monorail cost or they don't.
Regardless, make no mistake, a monorail would eventually pay for itself. As someone who manages a lot of trucks on the road every day for a business, it's a real problem that we're all going to face soon. When, not if, gas goes up to $4+ again, I don't know what's going to happen. This could be Disney trying to find a way to avoid that issue.
Personally, I don't think the monorail rumor has much substance. Not offense to the bus driver, but he's just a bus driver, and probably doesn't know much more than anyone else in the thread. As far ahead of its time as the monorail was when created, there is new technology nowadays, as already mentioned here, that I would think they would be more likely to go for.