How to dpm() the Drupal.settings.basePath var
Drupal.settings.basePath is a javascript variable, not PHP...you can't access it on the server side like that. 

I imagine the output you're getting in the message area is 'DrupalsettingsbasePath', which is how PHP would interpret Drupal.settings.basePath, along with a few PHP Notices complaining about the use of an undefined constant.

Assuming you're using a browser that supports console.log, to debug the base path just add the following in a javascript file that's included in your page:

console.log(Drupal.settings.basePath);


The variable will then be printed to your browser's console area.