Old comic books range wildly in price, depending upon market demand and the overall condition of the book in question. For serious collectors, properly pricing comics is vital to getting good value when buying or selling. Casual comic fans can also learn how to price old comic books as a way of gauging the comparative worth of a given book. It's also a way to make sure they aren't being ripped off.
Evaluate the condition of old comic books. That has a huge bearing on how the price is determined. Comics are gauged on their condition by a scale of eight different terms: Mint (MT), Near Mint (NM), Very Fine (VF), Fine (FN), Very Good (VG), Good (GD), Fair (FR) and Poor (PR). Pluses or minuses are sometimes added to indicate the book's placement within a given term (VF+, for example, means a book on the higher end of Very Fine).
Check the condition in which the comic book has been stored, if you can. Books which are sealed in mylar bags--and stored in a cool, dry place away from direct sunlight--will likely be in better shape and thus worth much more.
Educate yourself about the content of the comics, both in terms of the characters and story lines. Also know the writers and artists who created them. Issues in which noted characters first appeared, which involve popular or significant story lines or which represent the work of a famous artist or writer (especially if it was printed before they became well-known) are usually worth more than less memorable or distinctive issues.
Check pricing guides such as "Overstreet Price Guide" and "Comics Price Guide." (See Resources). They usually provide a good bell weather for the state of the market and which books are selling for what price. You can also monitor comic book auctions on sites like eBay to see what people are actually paying for a given issue.
Speak to dealers at your local comic book store or at comic book conventions in your area. They can give you a good sense of which books and characters are hot right now and what types of old books are selling for which prices. The more experts you can speak to, the better: the aggregate average of their opinions will probably be much more accurate than those of a single dealer or two.
Select a price which reflects the condition of the comic in question, its perceived market value as selected by experts and your own sense of what you are likely to get for it. Be prepared to lower the price if you wish to move it more quickly...or to raise it if demand for the book dictates.