nejprve musíš ty mysql dotazy vykonat:

<?
$spojenie=mysql_connect("localhost","root","");


$q = mysql_query("SELECT SQL_CALC_FOUND_ROWS SUM(`Mnozstvo`) FROM `naklady`");
$r = mysql_fetch_row($q);
$dodane = $r[0];

$qq = mysql_query("SELECT SQL_CALC_FOUND_ROWS SUM(`predaj`) FROM `predaj`");
$rr = mysql_fetch_row($qq);
$predane = $r[0];


$zostatok=($dodane-$predane);
echo $zostatok;
?>