Ok this is now sovled 🙂 and works well for me it does just a little playing around. Mediawerk thanks for help, I had a feeling that I would have to use js in so form. Little change to the code you provided but really help me a lot, thank you!
My code is below on html output I used and the JS.
Outputs the html
<?php
$rows = get_field('product_range');
if($rows){
echo '<select id="quantity" name="quantity" required>';
foreach($rows as $row){
echo '<option name="' . $row['product_quantity'] . '" value="' . $row['product_quantity'] . '" data-price="' . $row['product_price'] . '">' . $row['product_quantity'] . '</option>';
}
echo '</select>';
echo '<span class="price-text"></span>';
}
?>
JS used
$(document).ready(function(){
$("#quantity").change(function() {
var price = $("#quantity option:selected").data("price");
jQuery(".price-text").text(price);
});
});