Proving That Internet Advertising Is The Best

I need to justify to my directors our online advertising effectiveness vs offline advertising.
The major trouble is that we don't actually sell online (or data capture) but encourage people to visit

our showrooms. Thus proving effectiveness can be tough.

Could anyone point me towards any trusted stats / reports which show averages, for a print advertisement:

% of people likely to visit website for more information
% of people likely to visit store direct

Thanks!