Several of the responses thus far have mentioned a variety of costs associated with being a firearm manufacturer, only some of which are what could be correctly called "direct manufacturing costs".
Things like advertising, R&D, sales support, etc., while being costs of doing business, are not considered direct manufacturing costs. Only the cost of materials used in the firearm, cost of labor to fabricate, assemble and test the gun, and certain overhead costs (electricity to run the machine tools, cutting oils, etc.) are normally included in true "DIRECT manufacturing costs."
As to the original poster's question--such costs depend STRONGLY on both the firearm under construction, and the geography of the firearm maker.
Firearms made by traditional methods, in traditional factories, tend to be fairly expensive in terms of "direct manufacturing costs". Examples of these would be arms such as the M1911 pistol as built by Colt. That design is labor intensive in it's manufacture, and reflects a design philosophy that was present in 1911, when machine tools were less consistent, but human labor to had fit parts could be had relatively cheaply.
Firearms made by more modern methods, taking advantage of lower labor cost designs, are usually less expensive. A good example of this would be the Glock pistols. Glocks are, by design, virtually free of hand fitting, and thus labor costs to assemble a Glock is low, and the parts are relatively simple to make.
Now before everyone jumps up and down about how a Glock costs almost the same as a M1911, remember that I talked only of DIRECT manufacturing costs. Glock frames are cheap to mold (compared the cost of forging and cutting an M1911 frame like Colt does it).
Where Glock's are NOT so cheap is in the amortizing of that very expensive and complex injection molding tool that makes those frames for a fraction of the cost of an equivalent M1911 frame.
Once you start adding in INDIRECT costs like tooling amortization, then costs of more modern designs don't always appear to be so very much less expensive.