How to do simple addition to increase favorites count of my wordpress posts

You can simply do it by adding 300 when displaying count as shown in following code:

<?php echo 'TOTAL POST VIEWS: ' .(intval(wpfp_get_current_count()) + 300); ?>

and instead of echoing that values in function return it as shown in following code:

function wpfp_get_current_count() {
global $wpdb;
$current_post = get_the_ID();
$query = "SELECT post_id, meta_value, post_status FROM $wpdb->postmeta";
$query .= " LEFT JOIN $wpdb->posts ON post_id=$wpdb->posts.ID";
$query .= " WHERE post_status="publish" AND meta_key='wpfp_favorites' AND post_id = '".$current_post."'";
$results = $wpdb->get_results($query);
if ($results) {
foreach ($results as $o):
    return $o->meta_value;
endforeach;
}else { return 0;}}

For more information on intval() function refer this page.