Moxa Uport 1110 is little bit cheaper, but don't have RS422/485.
http://www.moxa.com/product/UPort_1110.htm
ATEN usb have worked for me. From Ebay it starts something like 14 bucks or euros. (Here price is over doubled at local store
http://www.aten.com/products/productItem.php?model_no=UC232A#.Vq3PAX97ORT