Another point that is forgotten is that stamp catalogs don't set prices but are supposed to reflect prices as they exist in the market. If you search for some stamps on StampWants or
ebay you may see 20 available, others might just have 10 or 3 or 1. Now which are more common? And if none is available, don't you think that the catalog might be wrong and that the stamp is actually worth more than the catalog suggests? I certainly do.
People who demand everything at half catalog are sometimes just ridiculous. Some stamps might actually be worth less than that (such as some of the readily available earlier issues of Britain and Canada) but some might actually be worth far more.