Not sure this will do what you want, but you might try setting the registry-saved system variable REMEMBERFOLDERS to 0.
When set to 1, AutoCAD will "remember" the last path used in each standard file selection dialog and use that as the default for a subsequent use. When set to 0, if you start the program by double-clicking on a shortcut item, the path specified in the "Start In" property of the shortcut will be used as the default for all standard file dialog boxes. (Previous is according to the 2014 Help.) The Help does not say what happens if it is set to 0 and the Start In property is blank, or what happens if you double-click on a drawing file to start AutoCAD (which I generally do not do, to assure that the proper ACA modules get loaded) or what happens when you open subsequent drawings. But you might want to experiment with it set to 0 and see if that works better for you.
David Koch
AutoCAD Architecture and Revit User
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