First off, do NOT upgrade the firmware. You will end up with bricked players. I do believe you can theme the menus to look closer to iPods, but upgrading will cause you nothing but a headache.
How much they will go for depends on the actual memory. After you find out, I would check out eBay and some other places like
http://www.dealextreme.com/products.dx/category.318
They have some 1GB and 2GB 3rd generation players. This should give you a pretty good idea of a price.