I should have been more clear. $500 million was the cost for the entire commercial contracts program, as someone else pointed out below that is ~$150 million per launch. Whether NASA spent $450 or $500 million on a shuttle launch is somewhat irrelevant, either way it is far more than the cost of private missions performing the same service.
The Shuttle was originally an Air Force partnership where NASA was supposed to also launch the USAF's payloads, including spy satellites and the like. The 20-ton payload created major compromises as far as reusability and then the USAF wasn't interested once the thing was built.
1 shuttle launch:
$450M (reality)
7 people
53,600lb to LEO
1 SpaceX dragon launch (ignoring for a second that it doesn't exist yet):
$160M (target)
7 people
7,300LB to LEO
So even if the Dragon V2 comes in at its targetted price and capacities (and all historical evidence says it very much will not do that), it'll take 7 launches of it to match the payload of a single Shuttle launch. Doing some quick math here... 7 * $160M is... a hell of a lot more than $450M
So pound for pound the shuttle is half the cost of the Dragon V2's estimated price.
And in terms of actually mission profiles take something like STS-31, the Hubble launch. Payload mass there was 26,187lbs. Assuming that weight could somehow be split perfectly into 7,300 chunks and assembled in space that's still 4 launches on the Dragon V2, or $640M.
It is not fair to compare the heavy lift capabilities of the Dragon to those of the space shuttle. The fact that the Space Shuttle does both heavy lift and crew transport means that whenever you want one, you also have to pay for the other. Of course you would not use the Dragon to launch the Hubble.
A fair comparison would be 1 Dragon launch and 1 Falcon Heavy launch which leave quite a bit of room for cost overruns before they surpass the cost of a shuttle launch while delivering twice the payload and the same number of people.
Lastly the total cost of the Space Shuttle program divided by number of launches gives us a cost of $1.5 billion per launch (2008 dollars), according to Wikipedia. While NASA lists the launch cost as $450 million, that does not include the majority of the cost of the program. NASA paid a comparatively small sum ($270 million) for SpaceX to develop the Dragon (Even if there was a second contract that I am unaware of, the cost still cannot approach that of the Space Shuttle).
