I used to use these programs, when you could earn $.10 per raw search, and prefills were allowed. Now with most programs, you earn less than that, and only on SERP clickthroughs. Goverture pays .02 for raw searches, but doesn't allow prefills - they are probably the largest.
This can be worth the time if you have a high volume of traffic.
The value of affiliate searches is almost non-existent. If you have other ways of monetizing your pages, the search box will cost you more in lost visitors (download time), than you will make with it.
Lets say you have a banner running on that page that is averaging an effect .20 cpm. That's taking into account defaults and probably a few low/no effective roi CPC campaigns. That's pretty much what is occuring accross all the banner networks right now. Some are even lower effective cpm's than that.
Say you put a 3k byte search box on it at $.02 perclick and get a high rate of 45 to 1 click throughs. That box has cost you money to have on the page. You will lose more visitors because of the download time than you will can gain in click throughs. It's a case of letting the banner do the work and reducing the noise factor on the page.
I don't know of any search engine affiliate program that will make you any money at this time. Most (like goto), will effectively cost you money to run. It's just not worth it.