Personally, I have not experienced any drawbacks between accessible sites and the ability to make money. Quite contrary, since accessibility is just a matter of ensuring that more diverse groups of people can access your site. And, with usability there haven't been any drawbacks either. Usability itself only ensures that your site is easy to navigate and find your ways around. But it does not limit your ability to highlight certain portions more than others.
Just make sure that the ads retain the main focus. Don't put the search box in a too prominent spot. Come up with a few different possibilities, look at them side by side, and choose the one which is most accessible while also retaining as much focus as possible on your ads.
That's where I'm torn. I want my navigation and search box to be easy to find and to be before the fold. But I suspect that more people will go on to another part of my site thorugh the search. Hopefully they will find ads that interest them on the new pages they find.
One thing I like about having my own search rather than using something like a google search that searchers will only find more pages on my site.
Do you know if we can put adsense on a search results page other than the Goodle search?
Anway I will take a careful look in a couple of weeks to see if the search box has hurt my earnings.
I need to look at the long run too. The more people look through my site and discover all I have to offer the more likely they will come back and tell others about the site. I get a lot of referrals by word or mouth or mouse. ;)