I was recently asked the following question.

Try using "position preference". You can set it to only allow ads in the #1 position, which should be the "green area". Adwords will manage the CPC so that your ads appear only in that position. You can limit the daily budget to control costs.

Read more about position preference from Google themselves

http://adwords.google.com/support/aw/bin/answer.py?hl=en&answer=36482