Disagree. You're looking at it from a purely technical standpoint. The terror value will eclipse the destructive / health power of a radiation weapon. The economy will not just falter, it will die.
That depends on two things:
#1 The target
#2 The ability of the government to respond decisively and correctly with mass DECON stations and a demonstrated ability to decontaminate the infrastructure of the target region so that basic services don't break down.
Also it depends slightly less on actions taken by the targets themselves to safeguard critical information in hard data and as software in different caches around the country. So if a Dirty Bomb is detonated on Wall Street if those systems have redundant data security and the ability to quickly evacuate the scene while clean up progresses, then the area will quickly shake off short term effects as a minor annoyance, rather than a cataclysmic event.