I took a look around on the internet, visiting various company and industry watchdog sites, looking for payloads deliverable to low earth orbit (LEO), and the launch costs associated with them. I found pretty good payload information. Launch costs are little more speculative, especially as some of the data was not quite current.
Update 9-25-12: I put this data on a more traceable basis in a later posting dated 5-26-12. That one supersedes this one. That same set of more traceable data I updated further, to include the projections for the new NASA giant SLS launch rocket, in a posting dated 9-13-12. That one supersedes both of these earlier ones.
I looked up some pretty reliable data for the Spacex family of Falcon rockets. Falcon 1 and Falcon-9 are already flying, and the cost data were available directly on the Spacex site. Falcon-Heavy is supposed to fly for the first time this year.
I found some pretty reliable payload data for a few members of ULA's Atlas-5 family. That's now a joint venture of Boeing and Lockheed Martin. The Atlas family was formerly operated by Lockheed Martin. The cost data came from a watchdog site, but I think it is fairly current. I have data for the -401, -551, and -HLV configurations.
I also found some payload data for a few Delta 4 configurations on the same ULA site. Delta is a Boeing operation, built in ULA facilities. I found cost data for only Delta-4 Heavy on a watchdog site, but it's about 5 years old. So, if anything, today's launch costs for Delta-4 Heavy would be higher still.
I processed these data as payload mass delivered from Canaveral to LEO, and as US dollars per unit mass of payload (payload mass divided by launch cost). The results are plotted here in metric units ($/kg vs metric tons) and in US customary units ($/lb vs US tons). I expected to see the "scale effect" operating (lower cost per mass at higher masses), and I did.
But, there are differences among the vendors. Remember, when assembling something large in LEO, it is the cost per unit mass that determines your overall project launch costs. Once the Spacex Falcon-Heavy is flying, it will be very hard to beat, in terms of unit cost and absolute payload tonnage. Judge for yourself ----
In US customary: