I'm wondering if two ad clicks are equally valued, if one of them found the site from Google, and the other came from a referral link, would that affect the revenue paid?
How about with a direct visit?
Ad clicks are definitely not equal in value. There are so many variables that can effect the cost-per-click (CPC).
Google answers why AdWords users get charged lower then their maximum CPC bids here.
The other main thing to consider is that Google uses a wide range of advertisers, so different ads will pay/cost different amounts.