About $20,000 :-)
More often, the titles relate more to the scale or complexity of the project managed, but I'd also expect Project Directors to play a more strategic role than the administrative one, which many Project Managers find themselves spending a reasonable amount of their time on.
Also, remember that a title is just a title - I've seen some Project Directors who are glorified administrative assistants and some Project Managers who are really executives.
Hope it will help!
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Thanks.