It all depends on what you mean by "cost". Generally speaking, the actual manufacturing cost of a product, be it guns or washing machines or blenders is 20-25% of the final retail price. So the manufacturing cost of that $1000 shotgun you see on the rack is likely $200 - $250. Every person that takes possession of that gun along the way adds approximately 30% to his cost before passing it along. The "cost" of that gun to the retailer you are buying from may be only 25-30% less than he has it marked at.
On top of all that, every manufacturer is going to have sales incentives, special deals, etc. for some of their top distributors. So figuring out how much mark up there is in a particular gun is problematic.
Remember one important fact - Every distributor, wholesaler, or retailer needs to make a profit on an item or they wouldn't be offering it. Let them make some money on the deal and you will both be happy.