You must use the sprintf function.
sample, if you have in ajax_onchange_prince.php this:
echo '<td class="main" align="right" colspan=3 id="display_price"><h3>' . $products_price . '</h3></td>';
change it to.
$products_price2 = sprintf("%01.2f", $products_price);
echo '<td class="main" align="right" colspan=3 id="display_price"><h3>' . $products_price2 . '</h3></td>';
Also add:
$products_price2 = sprintf("%01.2f", $products_price);
And change $products_price to $products_price2