JoZu

amzn_LowestNewPrice is not a numeric field because you have the price formatted ($XXX,XX OR EUR XXX,XX)
You can create a custom field contain unformatted amazon price (XXXXX) and everything work ok.

custom_query_fields.php

LINE 39:

‘order’ => ‘DESC’, //[ASC] [DESC]

change for ASC

Nice plugin for sort post by price range!