I'm looking on pricegrabber.com and found a few for $20-$60.

http://www.pricegrabber.com/search_gen.php?form_keyword=usb%20to%20serial&topc at_id=1&found=2&ut=4245255fb466c0a5

I don't see why the $20 one won't work. Please let us know of your results. Thanks.