I found a good article from Internet Retailer that states that visitors to websites that performed a site search converted at 7.54% vs. “all visitors” that converted at 2.79%. People who use site search know exactly what they want and you have one shot at displaying relevant results. It’s too easy for a visitor to “move-on” to the next site and try again. That report includes sites with a good search as well as sites with a poor search, so I’m sure the raw data shows widely varying conversions.
A site with a good site search can convert much higher than the 3x vs. a normal visitor as stated in the report.
If your site is small and your search is not very effective, you’re better off just removing the search function all together. Perform an A/B test with and without search and see how your site converts. Find a good free or inexpensive 3rd party site search tool. If you have the budget, try the Google Mini search appliance for about $3,000. I have yet to try it, but it looks very promising. They even have a 30 day return policy if you want to test it and develop an ROI model.