"When I run the numbers, the delta V saved or gained by a maglev launch is a small percentage of the total delta V needed for sub orbital transport. A single digit % of fuel weight savings."

What numbers?
Maglev launch speed of less than 400 mph?
And at or less than 10,000' elevation?

As I said with mothership using jet engines it probably isn't going to save much fuel- roughly, a commercial jet going on short trips uses about 1/4 of it fuel in the take off to cruising altitude and for a commercial jet to get to 10,000' and 400 mph is somewhere around 1/2 of the energy of a take off.

What I think is more significant is a mothership could have a higher gross take off weight- could carry a bigger rocket and get to the rocket to launch point in less time.

If you used same the SpaceShipOne as launched from Mojave and instead launched at from 10,000' at 300 mph it might take 1/2 of the time to get to the altitude to launch the rocket craft.

"The White Knight drops SpaceShipOne when they reached an altitude of about 50,000 feet (15 kilometers), and it takes 30 or 40 minutes for them to reach this altitude. Along the way they leveled out for some time while they checked all of the onboard systems."
http://www.richard-seaman.com/Aircraft/AirShows/SpaceShipOne2004/

So, if you reduce the 30 minutes to 15, that's significant if your suborbital leg of the trip is only taking less than a hour.

In terms of cost of the Maglev, you will need a track of a 2 or 3 kilometers- off the shelf that are currently in use- though need some modification in terms of the train cars and it's acceleration and deceleration is obviously more extreme compared to the trains, but a suborbital launch vehicle could have less mass than a train carrying over 100 passengers

"Well, feasibility study has calculated 1.6 Billion Euro, that is 27 million Euro per km."
http://www.magnetbahn-bayern.de/ENGLISH/english.html

"The doors shut, and the train accelerates like a skyscraper elevator, silently, smoothly, and rapidly, and by the time the last car leaves the station you already seem to be going 50 miles per hour. Four minutes of gravity-simulator-style acceleration later, in which the taxis on the parallel highway lose ground slowly, then quickly, then disappear as fast as if they were parked and you were whipping by at 220 miles per hour, you reach the peak speed of 270 miles per hour for the tiny 20-mile run."
http://www.slate.com/id/2115114/

Same train [I believe]:
"Two of the first three carriages for Shanghai have engines and each weighs 35.767 tons. They are 27.196 metres long, 3.7 metres wide and 3.7 metres high.

The third carriage is 24.768 metres long and weighs 32.746 tons, with an estimated capacity of 107-110 passengers.

Each of carriages costs 120 million yuan (US$14.5 million) and Shanghai needs 15 of them. "

So adding it up: 35.767 tons x 2 + 32.746 tons = 104.28 tons plus passengers: 150 lb x 110 = 16500 lbs
So, gross of about 225,000 lbs.

Hmm, not as much as I thought, so perhaps a 5 km track.

So, somewhere around 250 million dollars. And probably including all the bells and whistles of the spaceport, around 1/2 billion dollars to build plus whatever operating cost.

Now what could rationalize such an expenditure?

Let's say you could capture a market of 100,000 passenger over 10 years: 10,000 per year, 200 per week, 40 per day- a max of 8 launches per day.
This is mostly space tourism with say upper limit of $100,000 per ticket: a 10 billion market of which the spaceport charges 5% to launch- 1/2 billion dollar. Allowing that it doesn't recover all costs within 10 year due it's fees.
Because obviously the State will gain from having such an enterprise in it's state.
So, 1/2 million dollars of passengers per launch, give a cost of $25,000 charge per launch. So say the charge is instead $50,000 per launch of up to 100,000 lbs- $.50 per lb

According to astronautix:
"All technical data were considered confidential and the data presented here are estimated.

White Knight: Gross Mass: 4,100 kg; Empty Mass: 1,200 kg;
...
SpaceShipOne: Gross Mass: 3,600 kg; Empty Mass: 1,200 kg;"
http://www.astronautix.com/craft/spaipone.htm

So, the charge for SpaceShipOne launch would be about $10,000.

In fuel costs, Knight carries around 2800 Kg of fuel, so instead it might carry 2600 Kg of fuel, so say saving only about $50 of fuel. Saving 15 minutes of flight time would probably be more significant in terms of cost.

SpaceShipOne wasn't designed for commercial use- it's a prototype. But say you had something like it and were trying fit in 3-4 flights a day and wasn't running it's performance as near edge of it's capabilities- 2 pilots and 4 vehicles and maintenance team, and $50,000 to $100,000 per ticket.

Considering that one is going to have some amount of money for airport fees, considering that passengers may pay more money to spend less time getting to the actual trip they are really paying for. That being launched from a Mag Lev might fun all by it's self.

But probably more important aspect is the capability to increase the gross take off weight- the white knight of SpaceShipOne could carry a bigger SpaceShipOne.
But more importantly in terms of V-PRIZE, the white Knight of SpaceShipTwo, which is being designed for commercial use could carry a bigger SpaceShipTwo, perhaps therefore get somewhere near the in the ballpark of getting from Virgina to Europe