First of all - I know, don't do this. The application that put the URL in the column in the first place is MUCH better at handling URLs and ideally, you would just add columns for the GET variables you're after and have the application put those in. Parsing them out after the fact from a VARCHAR field is insane.
So now we're here and we need to do that thing that we said we shouldn't do. My approach is to use a Table Valued Function that only returns one column - the value of the variable or NULL if it can't find the variable in the querystring.