Google ads are something different from Google Ad Words.
For the former you'd need to get in touch with Google. I assume its some html code. Basically they send ads that at least are supposed to be related to the subject/interests of the site that's hosting them. There are a number of companies that do something similar. IIRC, GirlGenius.com uses "Project Wonderful" as does another web comic I read "Something Positive".
Google Ad Words are something that you pay for, that pop up your info when people perform searches on google that hit those words. That doesn't sound like what your client is looking for.
As to how much it'd bring in, hard to say. My spider-sense tingles, thinking it'd be better to negotiate custom ad solutions. Adsense is a bit like a "one-size fits all" model.