Mark Kleiman doesn't believe Rudy Giuliani quite the ISG because he was greedy. Instead:
What seems much more likely is that Giuliani joined the ISG because he thought it would help him in his quest for the Presidency, and then dropped off when he figured out that it would hurt him instead. Maybe he was quick enough — I never claimed the man wasn't shrewd — to figure out before the rest of us that the ISG would come down on a position that Bush, and more important the Republican primary voting base, wouldn't swallow. That made him decide to distance himself from the ISG by accepting rival speaking dates. Then when Baker said "Start showing up or quit," Giuliani quit, with a sigh of relief. The Republican base will tolerate someone with no coherent position on Iraq, or someone who doesn't know for Shi'ite about the actual problem of Islamist terrorism, as long as he makes it clear he purely loves killin' him a buncha A-rabs, but if Rudy's signature were on the ISG report Mitt Romney would wrap it around his neck: "my opponent, who seems to think that talking is a good response to terrorism ..."
This has a certain plausibility to it, but I'm not writing off the greed theory just yet.