General value of coins... how to know?

Hello. I was just wondering what is the easiest way to know a coin's value. I do check at ebay sometimes to get an idea, but sometimes the variety of sellers makes it difficult. Also I've asked in coin shops in my country (Costa Rica) for the vallue they give to certain costarrican coins... And well, of course I know the value would vary depending on the coin's quality, but is there like a fixed percentage?

http://www.ngccoin.com/poplookup/World-Coin-Price-Guide.aspx

  Fill in the first three boxes, or more, and screen will show latest values, for free.  

                      

Thanks. Though I went to the page and, from the coins I tried to look, most had no information at all.