you forgot to concatinate the last string ”, so youre missing a “+”
$('.customfield').val('<?php echo $userdata["id"] ?>,clicks,'+amount+','+cPrice+'');
though you dont need the last string bit cause its empty anyway so you can just
$('.customfield').val('<?php echo $userdata["id"] ?>,clicks,'+amount+','+cPrice);
and you can concatinate as many variables as you want to
in the docs you can read that .val(value)
value is a String an Array of Strings or a function,
so in your case you are passing a string
solved jQuery multiply vars inside value [duplicate]