I discover that when you click in to the Advanced Report for Search, you will see a line right after the earnings table that reads: "Revenue from AdSense for search may be offset at the end of the month by costs associated with performing searches." Does anyone got an idea what "performing search" means? To my interpretation, it means you get the rest of the earnings after the advertiser is satisfied with what your lead have brought him/her with.
No, it means that if your visitors use your search function a lot, Adsense reserves the right to charge for the service from your earnings. They will never charge you extra -- only what you've made from the search earnings. So you'll never pay out of pocket. This rarely happens, though, so I wouldn't worry about it unless you're doing 50 thousand searches a day.