Pesky PHP 5.3!
To resolve this, you will need to edit the file /administrator/components/com_nbill/framework/nbill.number.format.php around line 71, where it says:
$ret_val = number_format($ret_val, $decimal_places, ".", $thousands);
...replace that with:
$ret_val = number_format((double)$ret_val, $decimal_places, ".", $thousands);
The fix will be included in the next release.