Well it wouldn't have to go along all 4 freeways it takes to go from LA to DL, it could take a more direct route, going along the freeway route is just impractical. Plus, there's already light rail lines on some of those freeways.
As for the cost, well I don't know how Las Vegas did it, but whatever the cost is, if they wanted to do it, they could definitely figure out a way through different contractor bids and through state subsidies and sharing of profits to get it done.
Yes, there are bus lines currently, but thinking long term a monorail system would be more cost effective. You need to think 5, 10, 20 years out and the cost of fuel and maintenance and time stuck in traffic that the buses take to operate. Those costs add up to much more than a monorail system would. The monorail could be electric powered and get the power from the track itself. They could have solar panels spaced all along the track to provide power and the monorail would get energy from the track as it moved and any excess power from the panels could go back into the grid, and then when the panels weren't providing enough they could get energy back from the grid again. This isn't even factoring in the 'green' environmental benefits this would create for SoCal air as well as for the company. With the new climate change laws in CA, they might even be able to get some sort of write off or tax credit for doing a system like this that reduces emissions.
As for the Green Line problem, oh yea I'm well aware of that, I live near the Green Line. However as part of the 30/10 plan, they are planning to build a Green Line extension that will finally go to LAX in the next few years. And again, while most people who fly into LAX aren't going to Disneyland, this wouldn't just benefit those people, but could benefit all people living in LA, especially the west side, who go on a regular basis. And as for parking fee losses, it's not like this would be free, people could either pay every time they ride, or they could have a fee added on to their AP so they could get unlimited rides throughout the year.
It'll definitely be costly, and a big project, but in the long run it would be worth it, it would save the company money, make it easier and more convenient for lots of people to go to the DLR. Not just for the parks, but to see a concert at the House of Blues or even if they're going to events at the convention center or something. And it makes environmental sense.
It may not happen soon, but I think overall it's a good beneficial idea.. and where there's a will, there's a way. If Walt taught us anything with Disneyland, it's that.