I know some of our affiliates are using pay-per-click advertising to promote our services. Employment being one of the most competitive markets, bids can go as high as several bucks / click for certain keywords -- most particularly with Yahoo's Overture and MSN.
One of the rules for making profit with PPC is that your bids shouldn't be higher than a program's EPC conversion rate, otherwise you will irremediably be loosing money.
You might have noticed that every month, I mention how high the affiliate earning per click can go. This EPC should give you an idea of what's the maximum you can bid per click to maintain a positive return on investment.
While some affiliates do earn more than $2.50 per click, I would however recommend not bidding higher than $1 per click as a general rule.