I need a PHP script that can search for a specific file (with a box to type in criteria) within a certain web directory (just one directory) on my server. Are there any FREE or cheap search engines that i can use and customize to fit my needs?

I would recommend you to create a table in a database that may have the following fields :
id (unique identifier, you may use it to update data),
url (the location of the file on your server),
title (to identify it),
description (to describe what is in the file),
keywords (words you may use to search the file)
date (date of insertion).

And when you want to search for a file, the search will be made in the database. You will be able to see the details such as description etc.
And when you click on it, you will be able to go to the file you want by using the field URL that is in the DB.

It will be faster this way. All you have to do in the DB is inserting new file, you don't have to make your script do too much for nothing.