I sell C64 hardware & accessories on the US eBay site, and have found that a boxed 64 gets US$15-US$20. A 1541 will get US$5-US$10, probably closer to $5 because we're all inundated with them here.
Todd Elliott used to have a site tracking what Commodore computers were going for on eBay, but it hasn't been updated since 2004:
http://videocam.net.au/~eyeth/commodorepricelist.php
