How much is the Amazing Fantasy comic where Spider Man makes his first appearance worth?

Amazing Fantasy #15 is probably worth as much as you are willing to pay for it. However, it is currently selling (2010) at approximately $120,000 at the upper end of the scale; if you are willing to believe some auction houses. Even low grade copies are selling at a premium. I believe that there are about 1400 copies known to exist and very few of these are high grade copies. Like most valuable items it is a matter of supply and demand. I hope this is some help to you.