I know this is an old thread but I was having the same problem. The previous source code change for getResources doesn't quite work for the latest version of getResources (1.5.1-pl) but the latest version now supports sortbyTVType so I was able to make the change there. That it might help someone else, I changed
case 'decimal':
$criteria->select("CAST(IFNULL(tvSort.value, tvDefault.default_text) AS DECIMAL) AS sortTV");
break;
to
case 'decimal':
$criteria->select("CAST(IFNULL(tvSort.value, tvDefault.default_text) AS DECIMAL(10,2)) AS sortTV");
break;
