That sounds about right for a larger engineering project. Probably a little high, but just compare it to many other engineering tasks.

Completely new car design = $1 to 2 billion
Computer processor = up to $5 billion
New Airliner = $5 to 10 billion
Comanche Helicopter = $8 billion
Stealth Bomber or F-22 = $40+ billion

Cars and Airliners don't change that much and they still cost billions to design. Spacecraft aren't designed that often and it costs a lot more. I think a B-2 stealth bomber costs something like $250 million to build. But development costs were something like $40 billion. So the unit cost of the 20 some planes is about $2 billion per.

A common number used is $250,000 per engineer per year. This number includes salary, benefits, equipment, tools, buildings, management, etc.

At $15 billion that means they think the CEV will take 60,000 person-years of work to design and test. If development takes 5 years, that means they expect they will need about 12,000 workers.

To put it another way, lets assume the CEV has 1 million parts. That means each part will have one worker spend 3 weeks on it. That includes designing, prototyping, testing, and documenting. This doesn't include software, which is becoming one of the most expensive elements of projects. Integrating all the parts also costs a lot. You also have to build the machines to build the vehicles, set up plants, and train workers.

These billion dollar figures sound like a lot, but it costs a lot to do anything significant.