To see what the DesertXPress is competing against, I used Google Maps to plan a trip between Victorville and Las Vegas.
Distance: 198 mi / 319 km  almost all of it on I15
Time: 2h 59m
Speed: 66 mph / 107 kmh
Some people have accused Google Maps's programmers of assuming NASCAR drivers, but the calculated speed is a plausible bestcase figure.
Greyhound's buses have a scheduled time of 3h 15m, subtracting out their layover at Barstow. That gives a speed of 61 mph / 98 kmh, not much worse than the car figure. Possible "Casino Express" buses likely have similar travel times. The DesertXPress trains will thus beat the cars and the buses by at least a factor of two.

I suspect that the DesertXPress Victorville station will need bus service to the rest the Los Angeles area, so as to get transitdependent and transitpreferring people, and also people who are reluctant to drive all the way to Victorville.
To obtain a busperformance reference, I will now try to estimate the performance of those Amtrak buses that take similar routes, in particular, the San Joaquin Bakersfield buses. These buses traverse mountainous terrain that is difficult for trains to cross without expensive construction, so they may be good analogs for Victorville  LAarea buses.
Bakersfield  Las Vegas: 5h 45m / 294 mi / 51 mph
Bakersfield  Victorville: 3h 5m / 147 mi / 48 mph
Bakersfield  Pasadena: 2h 5m / 114 mi / 55 mph
Bakersfield  Los Angeles: 2h 20m / 112 mi / 48 mph
To be a bit pessimistic, I will choose 48 mph. Now for some LAarea destinations, using Google Maps routing and no intermediate stops:
San Bernardino: 35 mi / 43m
Anaheim: 75 mi / 1h 34m
Los Angeles: 81 mi / 1h 41m
Even that may be rather optimistic, since the DXP buses may have several stops along the way to their farthest stops. So riding a DXP bus from somewhere in the LA area will likely take longer than the DXP train.
I will now attempt to estimate the cost of constructing the DesertXPress line. The ParisStrasbourg line,
LGV Est, will cost about EUR 4 billion for 400 km, making EUR 10 million/km or about $20 million/mile. A distance of 190 miles yields a cost of $3.8 billion. However, land will likely be much cheaper in the southern CaliforniaNevada desert than in eastern France, which will likely reduce the cost somewhat. And the DXP team hopes to scrimp further by using the I15 rightofway as much as possible.
I now turn from the line to the trains themselves. It's hard for me to find numbers on the cost of a highspeed trainset, but using a rough guess of $1 million/railcar (derived from locomotive cost) and 10 railcars/transet, I find $10 million/trainset.
As to how many trainsets the DXP will need, that depends on how many it will run. If each train has a 10minute layover, then it can make a round trip in 3 hours, going each way in 1h 30m. So if the DXP has 1 train/hour, then it will need 3 trains to cover the schedule. With an extra one as a spare, that makes 4 trains. With the above cost figure, I find $40 million total. Unless I grossly underestimated the cost of a typical highspeed trainset, this will be much less than the cost of the line.