Populate Field From Previous Field

I have been trying to populate a Number field from a previous Price field in the same form. I know that population of one field from a previous field in the same form is possible, but I don't know how.

My email to priority support directed me to the forum instead. I've spent about 20 hours in the documentation and forums trying to figure out how to do it, and can't find it anywhere. The documentation is a bit vague (although I understand you can't put instructions for everything there), and any forum posts are either left unanswered ("nevermind I figured it out," or "just use jQuery"), or tell the user to search the documentation and other posts. ...seriously?

If someone can please explain exactly how to do this, or at least provide a link to a post that truly does explain how to do this (what the code should look like, where it goes, etc.), I'd REALLY appreciate it.

Thanks.

I finally got an email response that actually had an answer in it. Very simple, as long as you can find this code snippit.

First, the field to be populated must be on a subsequent page from the field whose value you want to call.

Second, the field to be populated must have the "Allow field to be populated dynamically" checkbox CHECKED, in the form editor, under the "Advanced" tab for that field.

Finally, paste the following code (customized for your form and field id's) into functions.php :

<?php

// update "108" to the ID of your form
add_filter("gform_pre_render_108", "populate_previous_page_data");
function populate_previous_page_data($form){

    $page_number = rgpost("gform_target_page_number_{$form["id"]}");
    $page_number = !is_numeric($page_number) ? 1 : $page_number;

    foreach($form['fields'] as &$field) {

        if($field['id'] != 15) // update "15" to the ID of the field you wish to be populated;

        $field_page = rgar($field, 'pageNumber');

        if($page_number > $field_page)
            continue;

        // update this to the ID of the field whose value you wish to call
        // '16.3' = example, first name of a Name field
        // '16.6' = example, last name of a Name field
        $field['defaultValue'] = rgpost('input_16_3'); 

    }

    return $form;
}

That did it for me.