In the US IP owners can also assert rights under trade dress law to protect the "look and feel" of iconic products and packaging if the design itself is distinctive enough to be associated with the IP owner. The VW bug, the Coke bottle, and the iPod are all examples of products with distinctive trade dress. 3D models of real world objects can infringe trade dress rights because it their inclusion in the game may suggest a "false endorsement" from the actual manufactuerer. The line between protectable trade dress and unprotectable utilitarian designs is not always clearly demarcated, however if someone with knowledge of weapons can look at your model and tell you that it is a colt 45, then I would say you have failed the smell test and should design an alternative.

As for typical pricing for a license to use real world products, it really depends on your skill at finding the correct contact and negotiating those rights. As noted air craft manufacturers know their IP is valuable and charge high rates. Some companies don't care and may license the rights for free. However having done some of this work in the past I can confirm that it is time consuming, frustrating and definitely not cheap. It takes a long time to track down contacts and even longer to get the paperwork finalized. If you don't have a significant budget to clear the rights to all your 3D Models, then I would suggest making generic versions of the real world objects and focus on the gameplay rather than authenticity.