Would be like this:
foreach($product as $dkey=>$d)
{
$col=0;
$worksheet->write($row,$col++,$d);
$total = 0;
foreach($date1 as $tkey=>$t)
{
$worksheet->write($row,$col++,$price[$d][$t]);
$total += $price[$d][$t];
}
$worksheet->write($row,$col++,$total);
$row++;
}