Is there a place online to find the value of old coins?

Answer 1

www.pcgs.com/values

Answer 2

ebay is a great way to find out how much old coins are actually selling for. Your coins may have a perceived value, but if no one buys them at that price, they don't really have that value. By seeing prices on ebay and how much they sell for and how quickly, you can keep track of current market values.
Local coin clubs will be another source of coin values. Local coin dealers at hobby shops can look over your collection and appraise your coins. The prices they quote may be lower than you want, but again, on ebay they may sell higher than quoted.