I have a setup like this:
I then have a helper function that simply returns the full path to this depending on my setup.. something similar to:
I will usually keep common routines similar to this in the same file and autoload it with codeigniters autoload configuration. Note: autoload URL helper for base_url() access.
$autoload['helper'] = array('url','utility');
You will then have access to asset_url() throughout your code.