TalkPHP - float problem

Code:

<?php

include 'conn.php'; 

$post=$_GET['post'];



$query= mysql_query("SELECT min(cost) as mincost, iqid, warranty FROM cost WHERE postid='$post' GROUP BY iqid, warranty") or die(mysql_error());

while($row = mysql_fetch_array($query)){

$iqid=$row['iqid'];

$mincost=$row['mincost'];



$doup = mysql_query("UPDATE cost SET lowest='yes' WHERE cost=$mincost AND postid=$post") or die(mysql_error());

        if($doup){

        echo $mincost.'&nbsp;&nbsp;'. $iqid.'<br>';

        }

}

?>

if the lowest cost is float, other lowest cost are not updating.
but if the cost is integer, it is ok.
could you please help me?