I got so sick of AdSense for search for several reasons. One reason is that the TOS actually forbids changing the code, and as it is, the search box looks pretty amatuerish right out of the box. Another, more important reason, is that it pays peanuts. Since I am not an elephant, this doesn't make me real happy. So I found a great alternative: Fluid Dynamics Search Engine http://www.xav.com/scripts/search/ It has a free version and a paid one. I used the free one, and it is a gem! It has an auto-install script too -- I used it with no problem. I find it much better for vistors too, as you can customize any aspect of it, like adding "most recent searches", etc., and it also has an advertising manager in its control panel too. So, you can customize the templates to show ads as you like. It has superior usage stats to Google's and lots more. It has tons of helpfiles too. Seriously, why should I let a vistor leave my site for chump change?
Actually i'm pretty happy with my adsense search. For the people that do not use it yet : https://www.google.com/adsense/ws-overview As for : Your payout really depends on the number of searches done, and the advertisers bid on that particular clicked ad
Yeah, I'm not too happy with my AdSense search right now... Maybe I'll look into the Fluid Dynamics Search Engine
Can also use http://www.digitalpoint.com/tools/search/ and keep users on your site (more control over ads, template, etc.)