At an analyst's meeting today, Bill Cobb (the President of North America for eBay) has said, "Pricing is a little wild right now. We have to focus on getting fair value.", in regard to keyword pricing for online advertising for popular search based ads (for example Google AdWords).
Cobb went on to say that eBay will keep it's online advertising in line with previous levels. He also mentioned that eBay is developing a proprietary software to make eBay a "better bidder" on keywords.
It's all supply and demand... and with more businesses turning to online keyword advertising, eBay is just feeling it the most because it's one of the largest keyword advertisers in the world.