frm_before_display_content

This function will sum all values in field 171, but it will only include entries that are displayed in your View. If some entries are excluded due to a filter, it will not include those entries. Just put [sum_x] in your View's Before Content. Replace x with the ID of the field you would like to sum. You must also replace x in the code below with this field ID.

add_filter('frm_before_display_content', 'dynamic_frm_stats', 10, 4);
function dynamic_frm_stats($content, $display, $show, $atts){
    if ( $display->ID == 1066 ) {//Change 1066 to the ID of your View
        $entries = $atts['entry_ids'];
        $total = 0;
        foreach($entries as $entry){
            $current_value = FrmProEntriesController::get_field_value_shortcode(array( 'field_id' => x, 'entry' => $entry ) );
            if ( $current_value ) {
                $total += $current_value;
            }
        }
        $content = str_replace('[sum_x]', $total, $content);
    }
    return $content;
}