Ebay's completed listings. However, be aware that some people will pay over face for regular clad. People have sold bicentennials for up to 2x face. The lesson from eBay is to take things with a grain of salt. Don't start hoarding bicentennials, but it is a pretty good price guide. Another good price guide is the Red Book. The only downside is that those are dealer prices, and you will get much less if you sell.