mahogany is more expensive because it is a protected species, being hard to grow and increasingly rare. even though mahogany is a dense wood, it is soft. it is also a very good tonewood. alder is mainly used on bolt on guitars, but remember that most of the les pauls have maple as neck through body. that means that the pickups are actually mounted on maple, the only mahogany is in the wings. obviously this is not true if you have a gibby with a mahogany neck. in fact, the first solidbody that les paul built, "the log" was a solid piece of maple that he later glued wings onto so that people would take him seriously.