Prueba esto:
Código PHP:
$cadbusca = mysql_query("SELECT *, MATCH (cupon_titulo, cupon_descripcion) AGAINST ('" . $busqueda . "' IN BOOLEAN MODE) AS coincidencias FROM cupones AND MATCH (cupon_titulo, cupon_descripcion) AGAINST ('" . $busqueda . "' IN BOOLEAN MODE) ORDER BY coincidencias DESC");
Saludos!