split-and-sum-values.js

<script>
jQuery(document).ready(function($) {
    "use strict";

    $('#field_xxx').on('change', function() {
        var total = 0;
        if ( $(this).val().length > 0 ) {
            const split_vals = $(this).val().split(", ");
            var total = 0;
            for (var i = 0; i < split_vals.length; i++) {
                total += split_vals[i];
            }
        }
        $(this).val(total);
    });
});
</script>