Three online sources to price coins are Pcgs.com, Cointrackers.com, and Ngcoin.com. They provide detailed information on pricing various coins based on condition, rarity and other variables. The Numismatic Guaranty Corporation, Coin Trackers, and Professional Coin Grading Service have all been in the business of grading coins since the mid-1980s.
Standards in coin grading can vary, but these websites claim to do the best job possible to ascertain the true value of items that they are assigned. The sites also feature online price guides that are updated daily, in addition to their own versions of coin encyclopedias that provide coin facts to help educate coin collectors.