Insert Ad's in free internet traffic.

Hi folks,

I don't mean to sound evil. I am trying to give free internet to my users and want to insert ad's in the internet traffic. Hoping the revenue will help support free internet. I want to use Nginx as a Cache Proxy instead of Squid. I came across mod_layout in Apache, which I understand, inserts ad's in Header and Footer dynamically. I like Nginx as it feels faster than Apache.

Would appreciate any advice.

Cheers