I've found pay-per-click advertising to be very beneficial. I have an online hobby store that has monthly sales of $6,000-$15,000 derived almost entirely by pay-per-click traffic from Overture.com, which was Goto.com.
The article below is from DM News and it's interesting from the standpoint the business being talked about gives solid numbers of what benefit they are getting from being associated with Overture and their pay-per-click model and the associated costs involved.
Hope you find it interesting.
Mike Winicki
Pay-Per-Click Boosts Sales!