They range too greatly. There are pricing guides available, just like for coins. I suggest getting one if you want to know. Stamps are usually only worth much money if they are in perfect condition or very very rare. They can range from 5 for a buck all the way to a million or so for the one surviving upside-down biplane stamp.
Currently stamps are worth .44 cents in the US to send a letter, as of Feb 2010. That's about two for a buck in mint condition or rough around the edges and five of those babies are.... Well you see where I'm going with this.... Makes me wonder if old stamps are worth it? Just saying...