A lunar-capable Dragon will require some modifications, but not as much as one might think. SpaceX claims that it was designed from the beginning for lunar return (the heat shield has to be capable of handling about twice the energy of entry compared to a return from low earth orbit), but the life support system will have to last long enough, and they may have to beef up the avionics against radiation. The last mission to the International Space Station sustained several “single-event upsets” when stray radiation caused some computer resets. The radiation situation will be worse on the way to and from lunar orbit, without the protection of earth’s magnetic field existing in low earth orbit. In addition, Golden Spike will have to develop itself, or hire SpaceX to develop a larger propulsion/service “trunk” to handle the longer mission and higher velocities. But given the latter company’s cost history to date (the Dragon was developed for anywhere from four to ten times less than the Air Force and NASA cost models would have predicted), there is no particular reason to think that such an effort will exceed the estimates. The key factor is stated in their technical paper:
Efficient operations, focusing on the job itself, rather than on the number of jobs created.
When NASA went to the moon over four decades ago, it was important to actually accomplish the goal of beating the Soviets there, but since then, Congress has been much more concerned with where the money goes than with whether NASA actually sends people into space, which is why NASA human spaceflight programs tend to run behind schedule and over budget. As SpaceX, Masten, Armadillo and others have shown, tremendous gains can be made with relatively modest resources when a team is allowed to focus on the goal, rather than where and by whom it is accomplished. Golden Spike will be operating in a similar goal-oriented environment.
The biggest barrier to their plans are not technical or cost, but whether they will be able to raise the money, particularly amid skepticism, justified or not, about their plans. As the old saying goes in space policy, no bucks, no Buck Rogers.