HttpContext.Current.Error

Hi,
 
I'm overwriting some code that handles callbacks so I can get exceptions logged in every possible way...
 
But callbacks are called very often on my webapp.
 
What I'd like to know is what is the performance cost to call HttpContext.Current.Error very often. Could somebody explain or point me a way to get this info?!
 
What I know so far is that it will return null if there's no exception on the webapp, but to return null, calling this will cost the same as throwing/creating an Exception (which has a significant performance impact) ?!
 
Thanks for the help!
 
 
...