I don't know, but here's how I think it works:
The supplier sells them to the dealer at a given price, which will vary a little according to the amount bought. The dealer then looks around at the competition in his immediate area and then sets the price accordingly, ensuring a mark up. As you can't buy mail order or off t'internet, like most things these days, that's a plus for the dealer of course. (well actually I think you can buy off the web from some places, but you still have to travel there and do the deal, so anything you save is likely taken up with travel costs). I don't know how much the mark up is, but I would hazard a guess that it must be in the region of 50% give or take for them to make a living. Incorporated in the mark up will be some "wiggle room". So that if a customer is a regular, or they are buying a pile of stuff, or there's no worn out useless wobbly-faced shotgun part exchange, the dealer has the option to give a discount, which of course will still be in the area that gives the dealer a good profit.
Then of course the dealer will direct you to the cleaning stuff, ammunition, rifle slips, bipods, moderators, mounts, scopes, reloading equipment, sandwich makers, toasters, etc. Some of which will have profit margins in percentages well over three figures.
The dealer then goes out and buys a new full fat Range Rover.