Cost Plus Fixed Fee is the grand holy of holies when it comes to government development contracts. It means you get to charge the customer whatever it costs to develop the system and get guaranteed a profit. The typical strategy for winning such a contract is to low-ball the bid then make up for it in overruns. It's a grand NASA tradition.

Those Boeing and Lockheed-Martin designs would cost incredible amounts to develop. Using the projected cost to develop the CRV (~ $500 million) as a guideline, just the orbital component alone would easily cost at least $4-6 billion. Each of the booster sections would cost at least $2-3 billion, too, and probably a lot more. Throw in the processing time necessary to get all those pieces ready for a mission and you've got a grand boondoggle.

The Northrup-Orbital Sciences proposals look a lot like what the Air Force has in mind. The orbital section appears much smaller than the Boeing/L-M proposals and that means it would cost proportionately less to develop. Throughout decades of aerospace development, the bigger/heavier the vehicle, the more it costs to develop and build. The N-OS proposal looks like a fleshed out and somewhat enlarged CRV. I'd estimate it'd cost somewhere around $2 billion to develop that part. Since it's a lot smaller than the other proposals, it doesn't need as much for the booster sections. One proposal simply sticks the orbiter on top of a Delta-IV Heavy. That one would cost the least to develop, which is why NASA will probably never go for it.

Like I said before, NASA should separate the heavy lift mission from the manned mission. Heavy lift can be done on a booster like the Atlas V, Delta IV Heavy, Proton, or Ariane V. You don't need to send humans in order to launch a satellite or space station module. The manned component should be sized to carry a crew of 6-10 and perhaps 5K-10K pounds of cargo. I personally think Northrup-OS is on the right track. They appear to be proposing a much smaller, simplier, and less expensive vehicle, which is why NASA will almost certainly never go for the idea.