External Search Options: users choice

I have searched the forums before I posted and did not find anything addressing this topic, maybe I missed it somewhere or it’s not available.

External search options. I would like to give my members/visitors of my site the option to either choose to search my site or an external search engine. Either by checking a radio button for either “site” or “external s.e.” or a dropdown menu etc… on the the searchform.

Has this been discussed by anyone in the past, or would this be to trivial to perform?
This would even be a plugin I would pay to have written if there are no solutions readily made.