I would probably do something like this.
if (have_rows('formule')) {
$high = 0;
$low = 999999; // set to something higher than the highest can possibly be
echo '<p>';
while (have_rows('formule')) {
the_row();
the_sub_field('prix_de_la_formule');
echo '<br />';
$value = get_sub_field('duree_prestation');
if ($value > $high) {
$high = $value;
}
if ($value < $low) {
$low = $value;
}
} // end while have rows
echo $low;
if ($low != $high) {
echo ' to ',$high;
}
echo '</p>';
} // end if have_rows
