The United States is currently building the F-22, a high performance jet fighter reputed to be the best air to air combat (i.e. dog fighting) plane ever invented by a large margin. The current buy is about 180 planes at about $100-200 million each, which represents a huge decrease from original planes, which is part of the reason that they will end up being so expensive. The complete F-22 buy will be done by 2008 or so. The F-22 basically replaces the F-15.
The next figher jet that the United States will buy is the F-35. This is intended to replace the F-16 used by the Air Force, the F-18 used by the Navy on aircraft carriers, and the AV-8B Harrier, a vertical landing and takeoff plane currently used by the Marines. (It also replaces the just about to be retired F-14 used by the Navy.) The U.S. plans to buy more than 1,800 of them, and many of our allies also plan to buy F-35s, most prominently among them, the United Kingdom, who plans to replace their aircraft carrier aircraft with the F-35B (the vertical takeoff and landing version of the F-35).
The high volume allows the F-35 purchase price projections (and they are just projections at this point, it is still in the late stages of R&D), at about $30-45 million dollars each. In contrast, the going rate for a used F-16 is nearly $60 million dollars.
The Bush Administration's lack of diplomacy, however, extends well beyond high profile issues, however. As a result, the United Kingdom is now thinking about backing out of the F-35 project, and may go with a European built fighter instead now. The result may be increasing costs for the remaining purchases.