In file catalog/admin/products_attributes.php line 64

replace

tep_db_query("insert into " . TABLE_PRODUCTS_ATTRIBUTES . " values ('', '" . (int)$products_id . "', '" . (int)$options_id . "', '" . (int)$values_id . "', '" . tep_db_input($value_price) . "','', '" . tep_db_input($price_prefix) . "','" . tep_db_input($reference) . "')");

With

tep_db_query("insert into " . TABLE_PRODUCTS_ATTRIBUTES . " values ('', '" . (int)$products_id . "', '" . (int)$options_id . "', '" . (int)$values_id . "', '" . tep_db_input($value_price) . "', '" . tep_db_input($price_prefix) . "','" . tep_db_input($reference) . "')");

More...