I second the 'Dungeonomicon' approach a lot of people are taking here.

Quote Originally Posted by sonofzeal View Post
Indeed. Because otherwise, anyone can Get Rich Quick (tm) by buying two magnifying glasses and a metal tube (about 201 gp), and selling it as a Spyglass (1000 gp).
I prefer to set up an assembly line for chopping ladders in half down the middle. Two magnifying glasses don't necessarily make a very good telescope unless you have the magnifying glasses cut specially; the lenses in a telescope have to be the right size and shape to work together well.