Given that the Silver Line is proving extremely useful by finally hooking up Tysons Corner, a major city center, to the DC metro system, and given the general uselessness of shaving a few minutes off the existing Baltimore-DC routes, I'd hardly call $10 billion for a maglev line a "steal".
You're right, I did mistate the length.
In general though, I think you're misunderstanding my thrust here. You're talking about value, I don't think this project is a good value at all, quite the opposite [1].
I'm just thinking about engineering and construction...
The Silver Line is a demonstration that building 23 miles of 45 mph commuter rail line costs at least $6.8B despite the benefit the benefit of 40 years, 100+ miles and 90 stations worth of experience doing the same.
Given that, actually building a first of it's kind 40 mile 300 mph rail line through the same sort of aerial and tunnel requiring [2] suburban counties around DC for 'only' $10B seems completely implausible.
1: https://news.ycombinator.com/item?id=8492525
2: http://www.baltimoresun.com/news/maryland/bal-maglev-train-i...
Perhaps, but it's right on par with the cost of the recent Metro extension which cost $6.8B for an 11.6 mile extension running on existing tech.
Successfully building an entirely new Maglev line for 'only' $10B would be a steal relatively speaking, but I don't see that happening.
More likely, I'd expect the planners want Government buy-in because they recognize how susceptible the Government is to chasing sunk costs and covering all the difference when the project predictably balloons to 5x what was proposed.
1: http://en.wikipedia.org/wiki/Silver_Line_(Washington_Metro)